Formal:
* Glance: A quick look.
* Gaze: A long, steady look.
* Glimpse: A brief, incomplete view.
* Scrutinize: To examine something carefully.
Informal:
* Take a look: A simple, casual way to say "peek."
* Have a look: Similar to "take a look."
* Spy: To look secretly.
* Snoop: To look around curiously or secretly.
* Catch a sight of: To see something briefly.
The best synonym will depend on the context and the tone you want to convey.