Sechel, you’re chasing phantoms; almost no one significant is a misnaged nowadays. It’s very rare; i know of about 5 talmidei chachamim, 3 are not famous, who are real misnagdim.
As for knowing if someone is a talmid chacham…the ohr somayach wrote his seminal work on the rambam before writing meshech chochma on Chumash, because, he said, anyone can write on chumash…he wanted first to establish his credentials in chiddushei torah on sugyos.
As an aside, the meshech chochma haa got to be one of if not the most lomdishe of popular chumash seforim, but i digress…
If a rebbe writes chasidus, how can you expect a litvishe person to automatically assume that they are a talmid chacham? They won’t assume he is or he isn’t. There’s no assumption that a chasidishe rebbe is or isn’t a talmid chacham from a litvishe perspective.