I just, (perhaps foolishly) purchased a United Pacific horn set- 3 trumpets. I have no idea how to tell if they work or if i just got a $10.00 paper weight. can anyone tell me what I need to do next. I tried to blast air into the inlet via an air compressor and just blew dust out of the bells. 
Welcome to the forum. I don’t know much about horns but posting its picture would help a lot. The other guys here might know.
Welcome. How much air pressure (PSI) did you use to test the horns? I’m not familiar with the United Pacific in terms of disassembly but if you can take the end-caps off the horns, you could do that and inspect the diaphragms.
The compressor was at 60 to 125 psi for the several attempts. The diaphragm looks solid no nicks rips bumps but I don’t know what I’m really looking at. Or if its an issue I shot air directly into the port without any levers solenoid valve ect
First thing, make sure you have a good seal on your inlet so that the only air going anywhere is into the horn and not leaking back out. 60-100+ psi would be enough to test any horn.
Secondly, check that you have a reasonably good seal between the diaphragm and the nozzle or seat. Most air horns work on the same principle - i.e. a thin diaphragm is pressed against a circular seat (like the end of a pipe) and air needs to force itself between that and the diaphragm to get out via the front of the horn. That action lifts the diaphragm temporarily and because there is kinetic momentum and flex it slaps back down onto the seat before it all starts again (think of bouncing a ball). That constant oscillation won’t happen at all well if you have a leak inside the horn.
With a horn in good condition you generally won’t be able to blow through it just using your lungs - so that’s the initial thing to test. Put a piece of hose onto the inlet, your mouth on the other end, and then blow through it. If you can hear your air just whooshing back out through the horn then you know you have issues. That can either be because of gunk or damage to the seat/nozzle, diaphragm, or any seals (e.g. between horn body and end cap). Some horns (I’m not sure if yours have this feature) can also be adjusted in terms of the seat pressure on the diaphragm. This is generally seen via an adjustment screw in the centre of the back cap; i.e. the screw pushes against the diaphragm forward towards the nozzle, thereby increasing sealing pressure. If you do have such an adjustment, AND you have air coming out without resistance, you can try tightening that screw slowly until you have a proper seal. I’d probably advise against doing so unless you know for a fact there are no other issues because over adjustment on that front can easily damage the diaphragm.
Realistically if you can’t make them sound, then try taking them apart and post some pictures.