A new and interesting one is miking the diameter of primers. If your headspace is OK, the growth in primer diameter should correlate with pressure for any given set of constant primer case conditions. The case is important because the shape of the primer pocket will affect the flow of the primer rearward, creating a small flange - this is the spot that increases in diameter. For an initial baseline, do some measurements of factory loads, and use the same brass and primer types for reloads.
Another excellent approach is the use of a chronograph. Try to achieve the same VELOCITY indicated for a specific load in a good manual using the same components. Once you achieve the same velocity, on a consistent basis, you will be very close to the listed pressure. I do this with every load I work up, and powder weight varaitions are significant from the manuals. The hard part is adjusting the manual's velocity expectation with YOUR barrel length. Don't try to duplicat the velocity from a longer bbl with a shorter one. I deduct velocity/100 for each inch in bbl length difference - for 3000 fps/100, deduct 30 fps for each inch you are shorter. For pistols, velocity/25 might work - for 1200 fps/25, deduct 48 fps per inch. Don't try to deal with length variations over 4 inches, and don't work from shorter to longer, ie, increase the manual velocity.
You can combine both approaches and develop your own primer diameter data, related to pressure, but ALWAYS trust the chronograph over the primer diameter. Never exceed max velocities, no matter what. If your velocity is 5% high, your pressure could be 10% over max! Conversely, if your primers are too big, but max velocity is not reached, assume that you are too hot.