In one of Rabbi Yaakov Hillel’s online lectures (I thing on learntorah.com) he said that a good way to test mekubalim is when they come to your city, ask them to give a shiur in whatever Gemara the locals are learning. Quite a few will back off. As Ruth Alfasi said above, a real mekubal needs a Torah foundation.
Speaking of Rabbi Yaakov Hillel and the Steipler (mentioned above). Rabbi Hillel said that the Steipler was a Tsaddiq Nistar. Whatever you know about him is just 1% of who he truly was.