So my car is leaking oil but only when it is running?

how do i know where it is leaking from? it used to leak a little seep but now it leaks a bunch. i started my car earlier and i let it run for awhile and backed it up to leave and noticed it was huge puddle of oil and it was streaming out. i was told it was the valve cover gasket but how do i know for sure? and also its a 1990 ford probe are they hard to change? its leaking down about the middle of the motor on the bottom you cant see any leakage from the top you have to get under the car to see it. its like between the oil pan and transmission?