How would you calc probability of opponents hands?

[Jon Poker]

If the board showed:
10h, Jc, 8h, 4h

If you are against 3 opponents, how would you calculate the probability that someone has:
- a pair of jacks
- 2 pair
- a straight
- a flush

In general, I am trying to figure out what equation or process one could use to mathematically determine the probability of certain hands based on the cards on the board.

Any help would greatly be appreciated.