LSD-detection reagents (typically Ehrlich and Hofmann) will allow you to qualitatively detect if there is LSD present in the blotter, as well as help you to rule out the main adulterants for this substance (i.e. 25INBOMe and alike). Unfortunately it is not possible to detect the quantity of LSD present in a blotter.
Currently, blotters usually contain 100 micrograms of LSD, which is a reasonable psychedelic dose (a microdose would be 10 micrograms, a large dose 200-300 mcg, etc.). Of course, during the first time using a batch of LSD it is advisable to start with 1/4 or 1/2 of a blotter and increase dose as needed.
In any case, if you confirm that a sample contains only LSD, you are pretty safe, since no LSD overdose has ever been reported anywhere to date. Again, this is why you want to make sure you are not getting any of the adulterants, which are considerably more dangerous, as they are amphetamine-like compounds with a more narrow “safe dose” range.
That being said, if you really want to know the quantity in your blotter, you may analyze your substance through Energy Control (https://energycontrol-international.org/). They are based in Barcelona and are probably the most reliable testing service out there, but they do charge a fee AND you will have to send them one of your blotter tabs exclusively for testing. If you think this is something you would be interested in please email them before you send your sample and explain your needs to them.
You may also purchase reagents for LSD detection by clicking on the links bellow :)