As a rule of thumb, tank gauges with 1.5" or smaller face diameters are 1/8 NPT threaded. These gauges are typically seen as the secondary gauge that reads tank pressure. Gauges with 2.0" or larger face diameters are 1/4 NPT threaded. Another way to distinguish gauges is to see if they have teflon tape. NPT gauges are have taper threads and require teflon tape to seal from leaks.
Anyone who needs to replace or upgrade their gauge needs to ALWAYS empty the tank if replacing the second tank pressure gauge as it is under pressure. You can remove the larger gauge without emptying the tank if you keep the tank's air knob closed when replacing it. If the gauge threads are parallel instead of tapered there will either be a dowty seal (bonded washer with rubber o-ring inside) underneath the gauge or there will be a hard delrin washer at the base of the threads inside the valve. Most Chinese valves use M10*1 parallel threaded gauges in their fill assemblies. Most of the European valves use BSP threads which can begin to thread into an NPT opening. It will leak and damage the threads. Don't try to thread NPT gauges into BSP threaded fill assemblies.
Airtanksforsale, Airhog, and Omega assemblies use NPT threaded gauges. I've had very good experience using Durachoice gauges from Directmaterials.com in Houston. Their gauges are well made, accurate, and less expensive than most you'll see on Ebay, Amazon, and most retailers. I recently replaced the tank pressure gauge on an Airtanksforsale fill assembly with a Durachoice gauge when the original was off by 200 bar higher than was in the tank.