I honestly have no scientific or exact way to measure chain wear.
I know what the slack tolerances of the bike according to BMW are supposed to be but I am clueless as to how to measure it or what it looks like. This seems really dumb given that this is not my first bike but i've always guessed and told myself 'eehh that looks Ooook I guess!?" when I thought it looked acceptable..
For the moment I aim for not letting the chain drag on the frame slider and still being slack enough (gaging by feel) that everything seems like its good.
I do use a Mitutoyo caliper to make sure the tension adjustment on the back wheel is even between the two adjuster bolts, that seems to work OK. I have much more to learn on the subject, no doubt.