For example: lets say that there was 2 sheriff deputies in mark vehicles sitting low in the dark at night while on routine patrol and a unmark or undercover unit with tinted windows and a civilian license plate happens to speed by the mark units, how can those 2 mark units or any other law enforcement mark unit(s) tell it was a unmark or undercover unit from a civilian vehicle of the same type without stopping it?
Guys I mean the unmark is the same as undercover just in another word. What I'm trying to say is the difference from a CIVILIAN vehicle and a umark one of the same kind.

by the licence plate.

They can't always tell. That's kinda the point. I've been pulled over in an undercover vehicle several times and I've been pulled over in my unmarked vehicle, also. I've pulled over undercover vehicles, also. There are certain stickers that officers can put on their personal vehicles that make it possible for another officer to identify them as law enforcement but these stickers aren't used on undercover vehicles. There's nothing that makes a true undercover vehicle recognizable including the license plate.

You can't tell the difference between an undercover vehicle and a civilian vehicle. These vehicles are usually on a lease agreement with a dealership and they appear just like a civilian vehicle. They are changed out and returned to the dealership periodically.

How do you identify your car in a hugh parking lot? You know what it looks like because you see it frequently right?

Those unmarked cars are part ofa fleet of unmarked and marked cars that all LEOs see every day. They are aware of which units are unmarked, and there is other identification information that I won't put here.
Will just let you guess what that is.

well trust me if they had a way of knowing and something went wrong they would say they did know. when you are dealing with police and gov. its what they want you to know. but i am sure there are ways to tell. but you would think there would be a list of under covers and where they would be, but with the all he bad cops out there you can't even trust your department so i believe when a cop goes undercover only the people working the case knows. well good luck and i hope you are not trying to get our help so you can tell if a cop is watching you. ( stop selling them drugs LMAO)

It's really not that hard to figure out, especially since most unmarked cars are crown victorias just like the marked cop cars.

However, the cops can tell which ones are unmarked by the license plate. If the cop is in a situation where they think a car might be involved with something they'll run the plate, if its an unmarked police car the computer will say so.