In increasing order of intensity:
1 Glimpse
Best done over a newspaper. Detection unlikely.
2 Glance
Still casual. Check she’s not actually a fella, then move on.
3 Goggle
Eye-contact territory, be careful. You can always pretend you thought she was someone else.
4 Gaze
Seduce her with your penetrating stare!
5 Gawp
Five seconds til she slaps you.