1994 3.4 firebird leak?

[Joe K]

I have 94 bird 3.4l that after adding no smoke stop leak in the car. Which was recommended to me to help me pass emissions. After doing so i noticed a LARGE leak where the oil was actually running on the pavement. I am fearing a rear oil seal... i am hoping for an oil pan gasket leak. How do a check to see what it is? I know its a very expensive job to do.
Mainly leaks when engine is running. Not really smoke at all. Only smokes a bit when its cold outside. I did a recent oil change on it. Oil filter is snug.

 

[melsg5]

safely jack the car up and look at the bottom of then pan near the rear seal with the engine running. You should be able to tell if it's the pan or coming from higher up, the seal?