A quick look at the Mega board pinout and the Ramps 1.4 pinout gives:
Serial port pins Port RX TX 0 D0 D1 1 D19 D18 2 D17 D16 3 D15 D14
Of these pins, the only pair on the Ramps 1.4 pinout diagram is Serial 2 (D17, D16). They are on Aux 4. If you have a display, these pins will be covered and probably used. You would then have to dig deeper to see if any of the other serial pins are available.
Edit:
I dug a little deeper and found the other serial pins mapped as follows:
D18 - Endstop Z-Min D19 - Endstop Z-Max D14 - Endstop Z-Min D15 - Endstop Z-Max D12 - Fan 2 D13 - MOSFET
D13 controls a MOSFET, but I’m not sure what that MOSFET is designed to control…maybe another fan. If you need serial pins, and you are not using the features listed, then I suggest intercepting the pin before it connects to the Ramps board.