in regards to your 76's make sure they have the S mint mark first of all, if not theres no chance they are Ag.
2nd you could weigh them, if they are > 11.34g they are 40%
3rd - do the bounce test. Bounce some of your clads on the table and note the ringing sound, next bounce your 40% (and 90% when you find some) and notice the distinctive clanging sound is much different than the clad. Now bounce youre 76's if they are S mints and compare.