The only way to find out if the thread is really good quality, other than being sure that the brand is reputable is by stitching and see how it behaves. You can take a length of cotton and look at it by magnifying it (use a mobile phone), see how smooth the twist is and how much fibre (fuzz is visible).

Technically there are multiple criteria the raw material, construction, type of finish, tenacity and colour fastness.

Does the spool have information on it, The brand, fiber, the weight, the dye lot number?