If you can't beg/borrow/steal some gauges, at least watch the compressor operate, and note how frequently it cycles. If the charge is low, it will tend to suck the low side down rather quickly before tripping the low pressure switch and turning the compressor off. Kind of a hard thing to guess at unless the refrigerant charge is more than just marginally low.
But if it's low, you need gauges to charge, anyhow. They're not as cryptic to read as many people think. You just need to know the low side number you're looking for...depending on the ambient temp and the particular system, it should be somewhere in the 25-40psi range. I cite 30-35 as generally 'optimal,' but a little higher is not abnormal, especially if it's super hot out.
High side is important, too, but I'd pretty much just advise looking at a chart and seeing what high side should accompany the low side you're seeing. If it's 'in the ballpark,' I wouldn't worry about it. There's also the speed at which the pressures change with the compressor turns on and off, but that's hard to get a feel for without experience. If it goes from rest pressure (high/low equal) to the compressor cycling back off in a matter of seconds, that's too quick. It should meet some 'resistance' in the 40-50psi realm or thereabouts, and the gauge will start just creeping down instead of the quick movement it makes from rest. If nothing else, you can grab a can of refrigerant and start slowly letting some in. Do it in bursts so that you can observe the resulting pressure without anything flowing from the can. If the low side continues to hit the same pressure, the charge is/was low.
