After you've selected Gear Down and you don't get 3 greens it's either, Gear not locked down or bulb/power failure.
QRH says if no 3 greens wait 40 seconds and if no EICAS messages or gear door lights illuminated (the amber ones near the greens) then you can assume it's the bulb.
However, within 40 seconds you can expect a Gear Disagree EICAS with a checklist that directs you the appropriate action (including operation of the alternate system.)
As a final belt and braces you get a Configuration warning (v loud alarm + Red lights + Red EICAS message) if either thrust lever closed and the Gear isn't down below 800ft RA or with the flaps in the landing position.