Some thoughts...

Go and speak to them. Act like you don't know that they are cops. Tell them you noticed that their car was there for a while and ask if they need any assistance. Dead battery? Need a jump? Need me to call a tow truck? Be a concerned neighborly person....

If they were watching you - you would probably never see them. They don't want to tip you off.

Or you could call the police and tell them that a shady looking vehicle is loitering in the area