You may want to look for open air Wildlife Management Area ranges. They are often unsupervised.

The range I go to doesn't check anything, and it's a "clean, well-lighted place." But they have an RSO on the line at all times, and they definitely watch you.

The time I went to sketchy possibly fascist industrial park range, they didn't even have an RSO on the back line where I was but once every fifteen minutes to peek in the door and then close it again.

You could probably get away with it at a sketchy range, but I wouldn't do it the first time you go. Even then, I might add a "I was curious, have you ever had anyone come in with a 3DP gun?" and see what they say.

I've never had anyone check my guns at a range.

Yeah that's going to attract attention for sure, in more ways than one. The ranges I've been to are unmanned.

The only time I've ever had someone check my serial numbers is when I pick a new gun up from the Public Safety Ministry or when I fly, but I'm not American so your experience could be different

Depends what state you're in. I think some states require serials on all homemade guns. In those states, private ranges are probably going to check. But t even then, they probably aren't cross-referencing to see if it's yours.

My state doesn't require serials unless you go to transfer ownership.

Like the other poster said, go to state gamelands if possible. The ranger sometimes stops by to see if you paid for your hunting/range license, but that's about it.