My first recommendation is take it to a repair person. Play it for them and tell them the problems (maybe a different one from the one you first took it to). Then, before accepting it back play test it and make sure it is working.
It's very difficult to lead someone through an exercise in identifying problems when they don't have the knowledge, experience nor tools to do anything.
But in order to identify individual joints leaks simply hold one joint in you hand, close all hte keys, cover up the bottom of the joint with your hand, and mildly blow into the top of the joint. The air pressure should back up and no air should leak out of the pads. Do not blow too hard as air pressure can open a few keys (this could be a problem too). Do this for both the upper and lower joint. This can help you identify where there is a leak but you'll have to do this repeatedly to identify which pad(s) is leaking.
With this technique you'll be able to identify individual joint issues and tell the repair person that. Depending upon the problem you may not be able to adequately fix it, other than if it needs an adjusting screw adjustment.
But report back if you have any air leak and exact which key it is.
You may want to also tell us exactly which make and model and vintage clarinet you have as clarinets are different and we cannot see what you have.
Many repair people who are not clarinetists often use "gorilla" hands and squeeze very hard, thus closing up most problems that regular players have.