If you have any kind of workshop, you undoubtedly have a collection of random nuts and bolts. It’s incredibly satisfying when you can dip into the stash and locate the exact size you need to complete a project. The second most satisfying is when you bring a nut/bolt of unknown threading to the hardware store, screw them into that gauge they have hanging on the wall, and determine the precise threading so that you can acquire corresponding parts.
You can actually buy that yellow thread checker they’ve got bolted to the rack at home centers. Prime Tools sells them for about $60.
I’ve also seen this white variant at mom-and-pop hardware stores. It’s made by S&W Manufacturing and, like Prime Tools’ version, is sold in the $60 range.
Both of those are wall-mounted and meant to live in a fixed location. If you need something you can throw in a bag and bring to a jobsite, S&W and other manufacturers also offer this centipede-like variety. These are typically far cheaper, in the $15 to $40 range.
Typically far cheaper, but not always. Woodpeckers’ Thread Detective features easy-to-read markings and a higher price point of $55 for a three-piece set (either metric or Imperial) or $100 for a six-piece set (both metric and imperial).
Of course, with a little patience you can always make your own thread checker. The wall-mounted variety is easiest. While the one linked above uses a simply label maker for the text, I bet you could make a variant with bad-ass integrated labeling if you had access to a laser cutter or CNC mill. In fact, I wouldn’t be surprised if one of you has already made such a thing. If you do, please post a pic and I’ll integrate good ones into the entry itself.