As a rule of thumb, if you're getting stuff for free that you would normally have to pay a subscription or premium for (TV shows, channels or movies), or if you're getting stuff that wouldn't normally be available (e.g. films that are currently only in theatres, and not available for streaming on non-premium sources) then the source (and so addon) is probably not legit.
Stuff that needs a subscription is usually OK, as long as the subscription is to the original content provider (Disney, Sky, Netflix, Amazon Prime etc) rather than some 3rd party reseller who may or may not have the rights to redistribute the stuff they offer (there are many shady IPTV providers out there who charge a subscription or other fee for what is basically pirated media that they have no rights to distribute).
A look on our
banned addons (wiki) list will give you some idea of the most well known miscreants, but generally just apply common sense. Also stuff in our official repo (or with established threads in our addon support sections) is normally OK. The stuff in the repo is explicitly vetted by Team Kodi, and stuff we allow to have threads here is normally trustworthy, even if it is not in the repo for other reasons.
If you were offered something equivalent in a pub car park or a car boot sale, would it ring true or not. Stuff that is too good (or cheap) to be true most often is...
