What makes scientists already so certain that Comet C/2019 Q4 (Borisov) is a comet and not an asteroid?
It's the appearance that is mainly used to distinguish comets from asteroids. As noted in the discovery Minor Planet Electronic Circular (MPEC), numerous observers observed extended diffuse emission (i.e it looks "fuzzy" with a Full Width Half Maximum (FWHM) greater than the field stars, or shows a tail) typical of a comet. In the case of 1I/'Oumuamua, deep searches with 8-m telescopes failed to record any evidence for any extended emission, indicating it was most likely a bare rock. The much brighter magnitude of C/2019 Q4 does play a role in this, making a search for extended emission easier. 

The distinction has become more difficult in recent years with the discovery of activated asteroids such as (6478) Gault, Main Belt Comets but also objects like 1I/'Oumuamua on very long period, comet-like orbits which don't seem to show any sign of emission.