Not an easy task.
Usually a problem found to measure I & Q.
One way is to have calibrated paths to a combiner, with a certain known and trimmable phase and amplitude difference between two paths: when the output is ZERO (a good sensitivity is achieved with spectrum analizer), the two signals are opposite at combiner input. You can deembed single paths and calculate signals amplitude and phase.
I hope it can help.
Mazz
p.s. maybe Sergio Mariotti can give you more help; he's really skilled in lab.