Hi GavinHi Gavin,
I find the most reliable thing to do is remove the wheels and take them to a garage to use a gauge. Normally I do the tow vehicle wheels at the same time, knowing then that I have the correct pressures on the van and the car.
Not the easiest way to do it but worth it. What price on safety?
P.S. Use a gauge that you have to pay for, these are calibrated and checked by the Trading Standards and are more likely to give an accurate reading.
Simon V