You know it from knowledge of mechanics / experience / Math + how it translates to in-game result. Almost all skills vary in terms of number of skill points players put into them because it’s dependent on
-
personal preference / being a try-hard / priorities
-
build
-
hc vs sc
-
activity (superbosses / main campaign / SR / Crucible etc)
Best to ask for specific skills in specific builds. Or you just making a build and a more experienced player suggests changes.