5 mafia is a very specific number for an assumption. A Townie most likely wouldn't know the precise number of any particular Scum faction in the game, so players that act like they know the number of members of a particular Scum faction may be Scum.
There's also information from Night Actions. We can only reveal said information three times though, so they should be applied very judiciously.
@Kiwipom how do the info sharing restrictions interact with spurious information claims by non Town actors?