737 has viewing windows so you can look through there and see if the stripes line up indicating that the gear is locked down.
Nose gear viewer is in the cockpit floor right at the cockpit door. Main gear viewer is under an aisle seat and has a place to peel the carpet back (if the interior is put in correctly). Good that you checked that the viewing windows and stripe were clean on your preflight.
No green light step one is press to check if the bulbs are good. If bad install new bulbs or swap with another light cap. If not get out the non normal and follow the procedure.
Did see a guy retract the gear once and one of the green light caps popped out and disappeared. Was missing for a while but it was found in his coffee a little while later.