Current induced on a transmisison line
Consider the microstrip line as an capacitor, with whatever is radiating as one plate and the microstrip line forming the other side of the plate (and negliect fringe fields, especially if you put the radiator close). If you could estimate the capacitance. And then the capacitance between the microstrip line and ground, you could model the noise. 

Or if you wanted to measure it experimentally  you could turn off all voltage sources and connect and oscilloscope probe to the microstrip line and the ground of the oscilloscope to the ground of the PCB and measure the noise.