I'd been familiar with names of some characters through other media eg Steve Smith has a Shazam T shirt that's mentioned in the first episode of AD.

Never heard of Deathstroke before MKDC, or Sinestro in GL's ending

Never heard of Hawgirl before GAU, not sure who else is confirmed. grew up watching New adventures of Superman (with Dean Cain and Teri Hatcher) and Tim Burton's Batman movies, it's no wonder those chars are the most well-known.