It's not that bad, often you *can* tell. If the verb ends in -aru, -oru or -uru, it's always a godan (-u) verb.
agaru -> agarimasu (not agamasu)
noru -> norimasu (not nomasu)
uru -> urimasu (not umasu)
On the other hand, if it ends in -eru or -iru, it's usually - but not always - an ichidan (-ru) verb.
sasaeru -> sasaemasu (but: kaeru -> kaerimasu)
miru -> mimasu (but: hairu -> hairimasu)
Again, I'd expect this to be basic stuff that's shown in the textbook really... And it surprises me even more that your teacher didn't point this out.