About a week ago I shot a small buck with a 250 gr Speer fired from my 35 Whelen. It went through the chest and exited.
I am concerned it may have overpenetrated.
How do I know for sure?

If there are 2 more dead deer behind it.

If that very heavy bullet left only a small exit hole, she almost certainly did not expand.

The bigger the exit hole the better I like it.

Few years ago I put a 200 gr corelokt thru the lungs of a small whitetail buck with my .350 RM. Very good exit hole, drt, poured the heart lungs out when we opened it up. Very effective, but my .308 isn't quite as hard on my ears.

What did the chest contents look like?

Thnx for the grin emoticon.

Jerry

Had something like that happen to me once. Shot a doe in thick rivercane and had another one run off with it. I saw them both collapse after about 40 yards. Lucky for me Alabama allowed 2 does per day back then. I couldn't even find an entry wound on the second doe, much less an exit. When I cleaned it I found the abdomen was sloshing with blood, and a tiny copper jacket fragment had penetrated a hindquarter and severed the femoral artery.