Jerry was a connoisseur who collected rare coins.
In this sentence, connoisseur is being used as a noun. "Jerry was a [...]" can not be followed by an adjective when used this way. Jerry was a red, Jerry was a stupid, Jerry was a grumpy. These are all followed by different adjectives and do not make grammatical sense. Connoisseur may also not be a verb because these following would not make much sense either: Jerry was a running, Jerry was a took, Jerry was a walking, Jerry was a jumped.
The actual meaning of a connoisseur is a person who has good sense or judgement. A critic when it comes to tastes in things like art, etc. Without knowing this definition, however, the above were ways you could figure something like this out next time.