Lay Opinion Evidence
Generally, a witness may only testify to observed facts from which the trier-of-fact may draw inferences. This includes matters relating to their "knowledge, observation and experience". The main rationale for this limitation is that the "ready-formed references are not helpful to the trier of fact and might even be misleading". The personal belief or opinion by a non-expert is presumptively not admissible.
Opinion vs Observation
An opinion is simply an "inference from observed fact". Accordingly, a mere claim of "that's the guy" in identifying a person is simply an opinion where there is evidence underlying the conclusion given goes to it's weight.
There is no blight line between fact and opinion.
There is some distinction to be made between factual observations based on their "knowledge, observation and experience", which amounts to conclusions that are beyond the familiarity of the trier of fact, and actual lay opinion. 
Where Lay Opinion is Admissible
Lay opinion, being an opinion of a person who is not qualified as an expert, can be admissible in trial where the opinion constitutes a "compendious statement" of the observations of a witness in relation to matters of common experience where no special knowledge is required and the opinion is so close to fact that it is impossible to separate the two. 
There is no closed list of subjects upon which lay opinion may be given.
Lay opinion should usually have the following characteristics to be accepted:
- the witness has personal knowledge of observed facts;
- the witness is in a better position than the trier of fact to draw the inference;
- the witness has the necessary experiential capacity to draw the inference; and
- the opinion is a compendious mode of speaking and the witness could not as accurately, adequately, and with reasonable facility describe the facts he or she is testifying about.
R v Collins, 2001 CanLII 24124 (ON CA) at para 17
WBLI v Abbott and Halibruton,  2 SCR 182, 2015 SCC 23 (CanLII) at para 14 ("Witnesses are to testify as to the facts which they perceived, not as to inferences -- that is, the opinions that they drew from them.")
R v Xiao, 2016 BCSC 854 (CanLII) at para 23
WBLI, supra at para 14
- R v Browne and Angus (1951), 99 CCC 141 (BCCA), 1951 CanLII 393 (BC CA), per O'Halloran JA
- Collins, supra at para 17
R v Le (T.D.), 2011 MBCA 83 (CanLII) at para 139
Browne and Angus
R v Smith, 1952 CanLII 116 (ON CA), (1952), 103 CCC 58 (Ont. C.A.)
R v Harrison (1950), 99 CCC 96 (BCCA), 1950 CanLII 414 (BC CA)
R v McDonald (1951), 101 CCC 78 (BCCA), 1951 CanLII 416 (BC CA), per O'Halloran JA
R v Yates (1946), 85 CCC 334 (BCCA), 1946 CanLII 230 (BC CA), per O'Halloran JA
R v Sitar, (1992), 78 Man.R. (2d) 101(*no CanLII links) at paras 5-6 (C.A.)
Ilina, ibid. at para 72
Graat, supra at p. 835
R v Hamilton, 2011 ONCA 399 (CanLII) at paras 259 -- telecom engineers testified to cell phone communication towers
Ilina, supra, at paras 64 to 66 - officer testifies to bloody trail suggesting "clean up" marks
R v Colpitts, 2016 NSSC 48 (CanLII) - fraud investigator testifies to stock manipulation
R v Graat,  2 SCR 819, 1982 CanLII 33 (SCC)
R v Montague-Mitchell, 2018 SKCA 78 (CanLII), at para 38 ("lay witnesses may offer opinions where they are 'merely giving a compendious statement of facts that are too subtle and too complicated to be narrated separately and distinctly.' ") R v Collins, 2001 CanLII 24124 (ON CA), (2001), 160 CCC 85 (Ont. CA), 2001 CanLII 24124 (ON CA)
R v Ilina, 2003 MBCA 20 (CanLII) at para 80
E. G. Ewaschuk, Criminal Pleadings & Practice in Canada, 2nd ed. (Toronto: Canada Law Book, 2002), at c. 16, pp. 246-47): ("A “conclusory” opinion may be given by a lay or non-expert witness, as an exception to the general rule, when the opinion constitutes a “compendious statement” of the facts the witness observed if the facts involve matters of common experience and it is difficult to transmit the basis of the opinion.")
Graat, supra at p. 835
R v Ilina, 2003 MBCA 20 (CanLII) at para 74
R v Parada, 2016 SKCA 102 (CanLII) para 26
Examples of Lay Opinion
There is an unlimited number of categories of observations that a layperson can testify to. Lay opinion has been found to be valid in relation to the following:
- the identification of handwriting, persons and things;
- apparent age;
- the bodily plight or condition of a person, including death and illness;
- the emotional state or state of mind of a person (e.g. distressed, angry, aggressive, affectionate or depressed, state of shock, fear, happiness); 
- the condition of things—e.g. worn, shabby, used or new; 
- certain questions of value;
- estimates of speed and distance;
- visual identification of persons in videos; 
- visual identification of persons in pictures;
- visual identification of persons in court;
- visual identification of vehicles; 
- audio identification; 
- voice identification;
- identification of a shoe tread, including the type of shoe associated with the tread 
- that blood stains suggest someone tried to "clean up". 
A lay opinion that is formed from observations in close association and over a long period of time should be given considerable weight.
R v Parada, 2016 SKCA 102 (CanLII) at para 25
See R v Graat  2 SCR 819, 1982 CanLII 33
CEA s. 8
R v Pitre, 1932 CanLII 69 (SCC),  SCR 69
R v Abdi, 1997 CanLII 4448 (ON CA)
See Watt’s Manual of Criminal Evidence (2012 ed), Toronto: Carswell: 2012, at p. 462
R v Falkenberg, 1995 ABCA 27 (CanLII), (1995), 95 CCC (3d) 307, 165 A.R. 16 (Alta. C.A.) at para 7
- R v Leaney, 1989 CanLII 28 (SCC),  2 SCR 393
R v Nikolovski, 1996 CanLII 158 (SCC),  3 SCR 1197
- R v Richards  2 CCC 19 (BCCA), 1963 CanLII 678 (BC CA), per Bird JA
- R v Izzard (1990) 54 CCC (3d) 252 (ONCA), 1990 CanLII 11055 (ON CA), per Morden JA
- R v Assoun 2006 NSCA 47 (CanLII)
- R v Williams, 1995 CanLII 695 (ON CA), (1995), 98 CCC 160 (Ont.CA)
- R v Grabowski (1983) 8 CCC 3d 78 (QCCA), 1983 CanLII 3579 (QC CA), per McCarthy JA
R v Murray (1916) 27 CCC 247 (ABCA), 1916 CanLII 363 (AB CA), per Beck J
R v Hill (1986), 32 CCC (3d) 314 (Ont. CA), 1986 CanLII 4722 (ON CA), per Martin JA
R v Powell, 2006 ABCA 267 (CanLII)
R v Lee, 2010 SCC 52 (CanLII),  3 SCR 99
- R v Ilina, 2003 MBCA 20 (CanLII) at para 64 onward
- see Re: Price v Spence,  O.W.N. 80, 2 DLR 592, 1945 CanLII 339 (ON CA), at para 17
Police as a class of witnesses are often permitted to give opinion evidence as to their observations.
8. Comparison of a disputed writing with any writing proved to the satisfaction of the court to be genuine shall be permitted to be made by witnesses, and such writings, and the evidence of witnesses respecting those writings, may be submitted to the court and jury as proof of the genuineness or otherwise of the writing in dispute.
R.S., c. E-10, s. 8.
A judge is permitted to engage in his own comparison of handwriting. He does not need to give notice to defence before engaging in analysis.
R v Cunsolo, 2014 ONCA 364 (CanLII) at paras 40 to 44