What i usually notice are a lot of hard little black things... often mishapen things that don't look anything like eggs.
I go to war... as soon as i see them. I'll spend as long as it takes killing them. Then any moth I see around the house is dead meat. My gf hates moths because she says they eat your clothes, I used to laugh at her.
...then one day. I notice my leaves getting eaten, dismissed it... then i notice one. an ugly green thing, or brown thing (usually just those two colours) wriggling along MY plants, eating away at the leaves. I'd use a piece of paper or something to pluck the thing off, then crush it and watch as all my prescious leaf spills out of its body. They think they're clever... you need to look really hard to see them.
One thing i did notice is that they tended to stay away from the THC... only eating the larger leaves, and stopping once they hit sugar. Maybe I eradicate the mo' fo's too fast. No pesticides btw... hey, i'm indoors ffs.
Just good old hands.