Nouns ending in -on are usually masculine.
Words ending in -tion, -sion, -son, or -aison follow different rules.
Is pantalon masculine or feminine?
lepantalon
The rule behind it
91.9% of -on nouns are masculine
In context
J'aimerais que l'on nettoie ce pantalon.
I'd like to have these pants cleaned.
Ces deux pantalons sont faits de tissus différents.
These two pairs of pants are made from different materials.
Mon pantalon est usé jusqu'à la corde.
My pants are ruined.