Do you mean starting from cold and dark? Battery off, engines off etc?
Use the “flashlight” until you start up or at least turn on you batteries.
*Don’t sit there on battery only for longer than a few minutes, you’ll drain the power and not be able to start the a/c.
Engine(s) already running- Most a/c have overhead lights you can turn on as well as panel flood lights.
Example: TBM has overhead for both seats. I turn on the copilot overhead only because I find the pilots overhead a little too much…personal taste.
Look up to see/find the lights 