No direct evidence as you would expect. We only have circumstantial evidence such as selection of topics to cover and general view regarding covered events, which align quite well with position demonstrated by intelligence services. It also has some questionable funding sources (e.g. National Endowment for Democracy and Integrity Initiative), but I would say it also nothing more than circumstantial.

One recent analysis of Bellingcat behavior (not in general, but using a selected case) can be seen in this video (in Russian, but it has good English subtitles): https://www.youtube.com/watch?v=uDhhCLyYCU4 Author of the video is a Ukrainian opposition leader, who fled the country during Yanukovich rule, got asylum in Europe, and since then lives there.

"is believed by many"

He already said no.