> Step 1 is to measure the mass of the planet. This is surprisingly easy. Just need to catch a view of some object in orbit around the planet (like a moon) or passing near the planet (like a comet). If you can a couple of good observations, preferable over several weeks, you can calculate the mass of the planet using newton's laws of motion.

Measuring the mass of a planet is actually very hard, but observing a moon tells you the gravitational acceleration, which is what we actually want here. So we just cut out mass altogether and calculate surface gravity based on the gravity that we observe pulling on the moon.