It's not easy sometimes. You can roughly go by the year sometimes, but then you have the guys who swap in anything just because they have the other engine laying around. Being a 351, it's "supposed" to be a 351w according to the year. But I guess someone could have put in a 351m or a 351c. The quickest way to tell the difference between the 351w and the 351m is the number of valve cover bolts, 6 for the 351w, 8 for the 351m.
Trying to tell the difference between a 351m and a 351c can be tough. Luckily they didn't make the 351c for very long, so you don't seem them very often.