How I can I tell if my engine is a 355 or if it's a 383 stroker?

Pretty hard to do from the outside,maybe a good engine builder in the know could measure the piston travel through a sparkplug hole and tell?
What makes you think it may be a stroker?

I saw an old CL add that had a 1980 Camaro Z28 Blue. It's around the same time and place the previous owner bought this one from. So it made me think. Could this be the same car or just a coincidence. But the add read 383 stroker. So I thought I would check to see if there is some way to tell.

Well I guess your best bet is to try and contact the prior owner/s to see what you can find out.

If you can get enough vertical clearance, stick a thin wooden dowel rod down a spark plug hole. Make a reference mark (lined up to a specific point on the head) on the dowel with the piston at TDC, and again at BDC. Then measure between the two marks. A 383 stroker would measure 3.75". A 355 (350 stroke) will measure 3.5" (3.48 technically).