2:1 favorite means that if the
hands are played for every 3 times, player A will win twice, and B will win once.
As far as knowing if you're a favorite and how much. Essentially you have to put your opponent on a hand. Then calculate how many cards will allow him to beat your hand.
Ex. two pair vs flush draw. Assuming that one of your two pair is in the suit of your opponents flush draw and you do not hold a card in his suit, he has 8 outs. 13 cards of each suit, four of them being "known" leaves 9 cards to make the flush, however; 1 of those will give you a full house.
Now this is not 100% accurate however, to calculate percentage of hitting, after the turn I take (#of outs x 2 + 1), so in this example (8 x 2 = 16 + 1 =17%) you'd be an 83% to win or roughly 2:1.
After the river, I normally figure about 2x the previous percent, you have two chances at 17% = 34% to hit the flush draw after the flop.
I think this is pretty close. But, could someone with a little more knowledge revise this for me?