If anybodys new to Mira HQ i will guide you with the bguide
First of all, go to doorlogs (if you have the run diagnostics task, do that first (its on the right), you can click on the space bar and go away, like the medbay task in skeld and go back later) stay there for id say about 30 seconds to a minute, if anybody goes North or Southeast without going Southeast they are an imposter, to get to North or Southeast they have to go southwest.
If you see anyone vent right in front of you dont immediately go to call an emergency, they could be doing a trick where they go to the vent near cafeteria and kill you, wait a few mins or report a body
You can go to the corner at the hallway above the north sensor right outside of greenhouse and office, you can see vents open and see who comes out. You can do the same with the area between the 2 doors entering balcony
Go do reactor/lab tasks (The place west to the sensors) last, even though its close to spawn it has a large hallway which doors close for about 10 seconds, with a vent, so its very hard to escape and a good spot for imps to kill
If your a imposter you could wait for around 2 minutes after the start, if anybody has run diagnostics you can kill them.