....If refrigerant is coming out and your ac is overfull you ac will not work well....
This is awful vague, and if you're stating what creek said, he was vague as well.
Refrigerant coming out of the system, may not mean its overfilled, as well, you can easily overfill an AC system without back flowing refrigerant out of the low pressure fitting.
When you connect and disconnect fittings, it is normal to get a little shot of refrigerant to vent, that is NOT overfilled. The high side, under a lot of pressure, it may seem like more than a little.
If you have cheap equipment, it may leak, even the low side (suction) pressure of the AC system is higher than atmospheric pressure, so some refrigerant forcing its way out of leaks, is NOT an overfilled system.
The real way to tell how much refrigerant in the system? A pro with expensive equipment can suck the refrigerant out and weigh it, then pump back in by weight the exact amount your AC system specifies. The other way, an amateur can do, is to use a set of Hi/Low gauges, and look at the Hi/Low pressure relationship against the specs for the AC system and the ambient temps. As well as the AC performance from the vents.
If the system is overfilled, hi side pressure will be much higher than it should be for the outside air temp, the AC performance should still be good, until the pressure gets to high on the hi side. If the system is way overfilled, than the hi and low side pressures will be too high and the AC performance will be bad. Way overfilled, the high pressure will likely cause the system to shut down, it could even cause a safety valve to vent the extra pressure.
If the system is underfilled, hi side pressures will be much lower than it should be fro the outside air temp, the AC performance may suffer, if the system is way underfilled, the low side pressure will start to go up as the hi side comes down and AC performance will greatly suffer. The system gets to low on refrigerant it won't even turn on.
...Also the cans area quick somewhat fix not a substitution for proper ac work. It should be vacuumed of all refrigerant. Then oil and 134a added back in also checked for leeks. Contrary to popular belief AC systems do loose refrig without leaks not a lot but overtime it happens. I work on a fleet of over 100 large trucks that run everyday we see it all the time they last for 4 or 5 years before enough loss has occurred to require recharge.
If I were you i would use the can it may help for awhile. When I had time or money I would take it to a local shop with a 609 certified tech and the proper equipment to test and charge your system.
He's right, the only way to even have a clue to what is going on with your AC system is too put a proper set of Hi/Low gauges on them and operate it on MAX comparing the hi/low pressures based off the ambient for the day.
You can find the gauges cheap on ebay or borrow a set, for someone that might use every couple of years, a cheap set will do.
Or take it to a pro for diagnosing. If there is a malfunction there can be all sorts of different subtle reactions the gauges will show, that require some experience to figure out.
Adding refrigerant with dye in it can help to figure out where the leak is. My leak was in the evaporator, and thus I never found the dye, but clearly had a leak.