sometimes you can see if it is singular or plural only by article or adjective in front of the noun or by following verb.. you have to know particular noun.
in this case. singular is "der Lehrer trinkt" and plural is "die Lehrer trinken"
Yeah you're right! The ending "-in" for an occupation etc. is a typical female ending: Lehrer / Lehrerin (teacher), Arzt / Ärztin (doctor), Bauer / Bäuerin (farmer), Spieler / Spielerin (gamer) and so on