Where NY still has the opto-rectal outlook is that we still don't have titles to any older vehicles.
Anything 1972 and older has a "transferable registration", which doesn't need notarization, contains no prior owner history, and no record of lien assignments.
Your local DMV will accept it, because their title specimen book contains a picture, document # etc.
One problem is that there is no way to determine when you buy the bike that what you have is real, because the bike may have been sold previously using a duplicate transferable registration.
Another is if there is a loan, mechanic's lien, bankruptcy, foreclosure, divorce settlement etc. against the bike you don't have "clear title" until that judgment is satisfied. This means first you pay for the bike, then you pay off the owner's loan afterward (which may have been more than the bike was worth). He may have borrowed $10,000 for the bike, paid back $1,000, you buy it for $5,000 - you still owe the bank $9,000.
A legal sale under these conditions requires that either the lien holder (bank) signs off on the sale (generally in return for at least some of the money), or gets all of the money (and the owner gets nothing - he still owes whatever remains of the loan).
How to tell?
1. don't trust the seller.
2. you have to locate the lien yourself (very expensive to pay a service to do this).
3. there are 62 Counties in NY, and (theoretically) it could be filed in the County Clerk's Office in any of them. There is no central record of liens.
4. places to look: County where he lived, where the bike was registered (different address), where he worked, where he lived/worked when he bought the bike, where his bank is located.
5. if a lien is not filed, it's not your problem !!!
6. there is no charge to search for the lien, and only a minor charge for a copy to take to the lien holder. County Clerks are usually located in or next to local Supreme Court building in the County seat (Monroe Co. is in Rochester, Westchester Co. is White Plains, Nassau Co. is Mineola, etc.). The larger Counties have an on-line search: you're looking for a civil action with your owner as the defendant.