There are a multitude of features that signify a 64.5 Mustang - but many features carried over into 1965 as FORD used up parts - plus there has been a TON of mis-representation of these features.
The KEY things I look for are:
GENERATOR - ONLY 64.5 cars had them and the special wiring needed
BUILD DATE - The build date must be before July/August plant change-over - but there are a few late build cars out there - I do not know of a FRIM date when Ford switched
COUPE / CONVERT - There is NO documented 64.5 2+2 I've ever seen or heard about - people "talk" about one or two - but they usually end up just being very early 65 production
VIN - I'm no expert on the VIN sequences, but you can research the ranges for each factory - they are known pretty well and will tell you the approximately when the car was built.
2 speed heater motors, certain body features and sheet metal changes like the hood lip, the radiator core vents, or passenger side shock tower, etc can be misleading - yes they are features of early Mustang production, but they are not always unique to the 64.5 alone.
I hope this helps.