You can tell for sure by simply unbolting the starter. A 6V starter will have a shaft diameter of 5/8", and when you push the shift lever to engage the drive gear, the gear will move straight ahead. 12V starters have a 1/2" diameter shaft, and the gear rotates slightly as it engages due to angled drive lugs on the armature and the drive unit. This minimizes the possibility of gear teeth failing to mesh when the starter pedal is pushed. If you're working with a car starter rather than a truck (which you've carefully forgotten to mention) the solenoid on a key-start 6V starter is bolted onto the starter housing rather than being cast into the nose cone.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!