Most of the slot machine tokens have some wording on them that says "This token is only valid at _______ Casino".

My question of the evening is... If I am at Mandalay Bay and drop a few Circus Circus tokens in the slot machine... AND hit
"THE BIG ONE".... could/would Mandalay refuse to pay me?

They would pay you. Casinos can accept other casinos tokens for gaming purposes only.

Also,

1. Is there a way to determine which were the last tokens put into the slot that resulted in that BIG WIN?

Only if it stickks in the acceptor.

2. When "cashing out" credits from a slot, It is not unusual to sometimes find tokens from competing casinos mixed with the
ones from that casino.
Therefore - IF the casino COULD void a Jackpot based on the tokens used to win that Jackpot - should I look over each
token I drop in a slot to make sure it is from the casino I an playing in at that time?

NO

Actually many slot acceptors will not accept tokens from other casinos. You can take your foreign tokens and exchange them for tokens from the casino you are in. Gaming Regs say a casino may not give you cash for foreign coins but many will.