As russell king mentioned they do make gauges specifically for loaded tires. Milton industries air/water tire pressure gauge is one that is made to check loaded tires. I have one for my tractor, its tires are filled with calcium chloride. and that gauge will measure the pressure with the valve stem at any position. Any fluid that gets into the gauge will make a small mess but a rag or towel over the end when you remove the gauge prevents the mess. When i am done the gauge is cleanable to remove any calcium chloride to prevent it from corroding the gauge. I also have a very basic brass air chuck to inflate the tire if needed that can be cleaned easily.