Hopefully this doesn't seem an obvious response, but to my mind there seems to be at least two schools of thought regarding meditation. These schools are not mutually exclusive, FWIW.
One school seems to see meditation as an end to itself, the other as a tool towards some other end. I can't speak to the former, but for the latter - one approach you could follow is to identify experts who have written on the area of improvement and who prescribe meditation as a tool. For example, if you're using meditation to improve willpower, then a book on willpower will help you identify whether your meditation techniques are improving your willpower [1].
[1] For example, "The Willpower Instinct" (K. McGonigal) discusses the use of meditation to improve willpower, as well as the 'state of the art' on the subject, so it provides you with some tools you can use to evaluate whether you're making improvements or not