Then I can just say "I think someone can prove that [slander statement]" and I'm safe? Without seeing any evidence how can you tell if it's slander or not?

Hm. The definition is "Oral communication of false statements injurious to a person's reputation." So really, if it injures someone then its slander. Sounds like your initial idea was right - even implying wrongdoing could be slander.

The key word here is "false." Saying "X is being accused of" is or "X allegedly" is not a false statement, and that's why the qualification matters. Nothing said in the post(s) above was false, and therefore none of it is slander.