There isn’t any that I know of by default. But you can install the “SHIFT + Z” inforbar from flight.to that has a Simrate indicator.
Alternatively: just press the Reduce Sim Rate key a few times until you can be sure that you have reached the lowest sim rate setting and then press the Increase Sim Rate key twice. Since there are only two settings below the normal sim rate (1/2 speed and 1/4 speed), pressing the Increase key twice, will set it to 1, which is real time.
Here is the Shift + Z thingy: