We were recently discussing different ideas for readily available calibration targets.
You can see the tracks on a CD with 20x objective (just) or above. In transmission you need to peel off the foil. Put tape on the CD, then pull up. May need to score to get an edge to pull up. Blank CDR are good. The tracks are on the side if the disk that the foil pulled off. You may need dust or a pen mark so that you can focus as the tracks are low contrast and hard to see except when well focused. DVD and blue-ray construction is different and this does not work.
There is a standard CD track spacing, but I have not looked at how much variation there is, so whether this is a good reference.