Yes a 20 ga is perfectly fine for hunting turkeys. But like qandilamont said, it depends on how it's choked and how it patterns on paper.

Find out what choke the barrel is and if it is full you may very well be ok. If it's modified or improved cylinder you might consider having screw in chokes installed so you can use a dedicated turkey choke. If I had to guess on how it;'s choked I would say it would be a full but double check to make sure.

Next get a few boxes of a few different loads and go shoot them on a patterning board. Choose the one that has the tightest pattern and is centered at point of aim.