Have a crankset only a year old - the big chainring seems to have a slight wobble to it when I pedal. So I checked the chainring bolts - and sure enough, one seems to be "tight", but just spins when I try to tighten it. The other 4 are solid.

Is my best course of action to just replace with a new bolt?

Side note - is there a good way to determine if my large chainring really is slightly bent for having a loose bolt for awhile?

Thanks.

You need to hold the nut from the back while you tighten the bolt. You might want to loosen it and remove it and check the thread then reinstall. They work bear and don't (usually) need to be held if the outside of the nut and the recess on the chainring are dry, while the thread, and bolt side are oiled.

I use abrasive paste (lapping compound) under the rim of the nut to improve traction and never need to hold it making the job easier.

If it is a bolt and nut pair, sure, just replace both. Should be fairly inexpensive. If you are feeling wealthy, buy a whole new set of matching shiney new ones.

The only way I have ever checked for chainring trueness is to lay it on a known flat surface and inspect.

I'd recommend replacing all of them with Truvativ bolts. They require 5mm and 6mm allen wrenches, eliminating that annoying spanner type that Shimano supplies.

Bolt spin will be a thing of the past!