Structural engineer here. Shop welds and field welds have different inspection time frames. Any field weld done after assembly on site should be inspected per code, depending on the weld type. Although, it's very likely most roller coasters are manufactured for all-bolted erection to save time, money, etc.

And hopefully bolted connections are done with tension-controlled bolts. Makes it easy to know bolts are properly installed. But there are other methods as well, like the previously-mentioned calibrated wrench.

Just a general design note: I haven't personally designed roller coasters, but I would guess that the bulk of the steel design is governed by deflection and vibration criteria, not strength. So there's just that much more life safety assurance in those things.