I am not fine with authors cutting corners and would like explanations for such stuff, I'm a lore nut. I need the lore.
As for how I do it, the characters just sorta wing it and hope for the best. For a book I'm planning to write there is a thing called a looking glass, a scouter like eye piece or futuristic googles or head wear that allows you to measure the amount of aether in the air mostly used to determine cosmic phenomena, but can be used to measure characters power levels. I'd not exactly accurate 100% and plus it's expensive so not many people use and a lot of people that do use are either well off or fashion their own out of old parts that's even less accurate.
My thoughts on appraisal skill is I'm not a fan of it, most of the time it's not explained well and most just opt for the system route without explaining how the system strained that information or who made it work in a way that intelligent life could even comprehend the information. A lot of times these are never brought up which irks me, cause I got questions that I know would never be answered.