ask them things about their mythology! if you call upon hades, you could ask him if he’s the god of death! if he is truly hades, he will say no.
Now, onto how to test them! this is something i recommend doing every time you call upon them to make sure no entity interferes. if an entity gives you a high position (ie: priest, oracle, high priestess) before you have even worked with them for a year, definitely question them! if they also tell you things that sound very unlikely, or even impossible, that is also a big reason to question them. Tricksters also like to make the people they trick feel special. if the entity you work with does any of these, they are most likely a trickster and you need to test them! (i will tell you how in a bit) Some of the common signs of tricksters are: them treating you like a friend, being mad if you question their status as whatever they say they are, if they only want offerings and never want to bond, and demanding offerings.
even if you only experience one of these, please check to make sure! i will also tell how to know for sure in this section of the post. There is a range to how good a trickster at tricking you, thus someone may experience one or two of these with a trickster.