By journalists, prosecutors, House committee staffs, etc. If something could be found, I would think it would be damn near impossible not to be abandoned by even his most ardent supporters. While nothing that's come out thus far (e.g. Mueller report, not releasing tax returns, racism, etc) has shaken his base, being tied up in this mess could change that. I dont think it would necessarily require illegalactivity on Trump's part. Even if they were close friends, frequently socialized, or had business dealings, that might be sufficient. There's no way you could be in Epstein's orbit and not know he was a sleaze bag.