New tranny,,,Possibly!!Bad Gasket/Seal More than likely!!

I got this car from my step dad. Unfortunitly he passed away in august, before i could Thank him for the car. I didnt get to talk to him, but my mom did. He told her that he got a new transmition and Radiator put in it acouple weeks before. Sure enough, a brand new radiator is sittin under the hood. But about the tranny.
When i went to go move the car from his house i noticed tranny fluid(Redish) puddled up under the car. The transmition slips abit now, but i'm wondering if its because whoever put the tranny in, did'nt align the gasket properly. It doesnt slip all the time just some. Could Low trans Fluid be the case? I checked it and it's very clean fluid. I'm really thinking that the gasket is bad since the big puddle of fluid.
What do you guys think?

First of all fill it up as required, then get it up in the air, at a shop preferably, and see what is leaking, basic steps here.