AC - 6 Deck
I just played the entire boardwalk strip at AC from Taj to Tropicana. Everything was 6-deck so if your not sure thats probably the safe assumption. In fact I was surprised how little variation there was from playing one casino to the next.
I noticed that usually there's a sign hanging over the pit, or it's written on the table or somthing. There's also a colored sign on everytable listing the table rules, payouts and betting limits. I'm 99% sure it says how many decks are in play. They like to hide it behind the ashtrays, but it's there and you have every right to read it before putting out your money.
Any notable exceptions were also clearly signed. I think I saw one 4-deck game, some single/double deck and lots of wierd blackjack variations.
I saw lots of people asking to see the burn card and no-one took notice, so I doubt they would care if you asked how many decks they were using.
I never saw any rules against mid-shoe entry, although sometimes it annoyed the other players...
~Raven