I was working at a bike shop where we had 3 mechanics and about a dozen different chain checkers. Each one had their favorite, but very often I'd walk back into the service are to find 2 or 3 guys standing around one bike trying each checker in turn and debating the results.
Some frustrated Googling led me to this page and the Shimano TL-CN42. Lots of testing and comparison have convinced me that it is currently the best.
I've had a few mechanics complain that because it is Pass/Fail they don't know how worn the chain is. But these same mechanics are happy to waste time comparing the Rohloff, Park, and fancy digital KMC chain checkers over and over again and debate which is more accurate so...
This is my favorite park tool chain checker. The other one they sell has this stupid pin that gets bent and then the measurements are off. This one has a 0.5 and a 0.75 side. If you can't get it to fit in your chain - keep riding. If the 0.5 fits, you are almost there. If the 0.75 fits, it might be too late. Simple and trouble free. Will last a lifetime.
Found a picture online, right click, save picture location. Then in pinkbike click on insert photo (not insert pinkbike photo) and copied the address. Presto