Re: How to measure countersink angles of countersunk holes in sheet metal panels
I'm not sure if you or anyone else has looked at this thread for years, but I may be able to give additional help or advice. I currently work in aerospace, and we do both countersinking (machined) and 'dimpling' (or 'punched' as it has been called here).
Checking either can be straight forward. We often use 100 deg countersink gages to check the outer diameter, but usually our countersinks are 100 deg (although usually the customers may allow +/- .5-1.0 deg tolerance), depending upon the customer drawing and/or fastener callout. Since we typically don't worry about the angle, we don't use tools for checking taper angles that often, but there are gauging tools if you don't want to resort to using ball gages.
If you just want to verify the angle is not another standard angle, there are blade templates/gages for checking them. If you want to know the actual angle, there are some two-plunger gages (similar to single-plunger countersink and chamfer gages) that'll measure the angle, but often you'll need to customer order these. Ball gaging and mold with an optical comparator are often good (we used molds with optical comparators for checking large proprietary buttress threads in the oil & gas industry), but you could also use a plug gage (go-/no-go gage). If you know the fastener to be used, and you're worried about depth, I'd check the fastener to spec and place it in the taper as previously suggested... if concerned about depth precision, there are rivet head/flushness gages (essentially plunger dial indicators mounted on a three-point base) for checking smoothness.
Any of these should work, depending on what you're actual concern is with your supplier and your customer requirements.