Heated beds are just giant resistors. They don't have a polarity. The only thing that will matter is an LED that indicates if it's powered or not is if the LED is installed. If it is, then it matters only with the polarity of the diode. If you get it backwards, nothing bad happens aside from it not lighting up. So try both ways and there shouldn't be any issues. The SMD LED should have a mark on it (although if it's already soldered it may not be visible) indicating the cathode, which goes to the negative connection. Some boards also have two LEDs of opposite polarity direction so that it doesn't matter which way you hook it up.