Psi that the bottle holds? or psi through the gun?
Because both should be shown on gauges, gauge on the bottle is how much is in the bottle, gauge on the gun (if it has one) is the psi through the gun
Well without gauges... i should think you would find it pretty hard. lol
but yes mate, without a gauge you cant.(to my knowledge) unless when your filling, (if the fill rig has the function of reading the bottle PSI) and you fill the bottle and the gauge reads lets say 2500... thats how much you have in mate. but not pricey for a new reg for your bottle or indeed a whole bottle. worth the investment for a fiber wrapped one (if you have the cash)