There are a few prominent street fights Bruce participated in that have a few eye witness accounts, most notably his fight with Wong Jack Man. He apparently engaged in one against a Japanese martial arts instructor which lasted seconds and also had one on the set of ETD. There are numerous other examples that can be discussed.
Its a double edged blade with Bruce, especially because he died so young. Hes the Godfather of cinematic martial arts and was without doubt an exceptional practitioner. As such people will call you a jealous hater for saying he was nothing more than an actor, and others will say youre a fool for believing an actor could actually fight like him.
I personally believe he was legit, but thats my own opinion and i dont expect anyone else to agree with it.