Leaking kegs are just part of the business as a home brewer. They definitely can be frustrating but are well worth taking the time to figure out. When tackling any of my kegs leaking issues I will first disassemble and clean the keg, and then be sure to use ample keg lube when putting the poppets, dip tubes, and posts back together.
In order to save on usage of CO2 I usually fill the keg most of the way with water, put the lid on, and then hit the keg with 30 psi. Not only does this help seat the gaskets nicely, but it also makes it super easy to find leaks. After I hit the keg with 30 psi, I remove the gas line and spray super soapy water all over the main lid seal, base of the posts, and on top of the posts where the poppets seal. Make sure the water is very, very soapy to ensure that when sprayed it comes out almost as pure suds.
If you keg has any leaks at all without the hoses connected you will easily see the bubbles forming in the spot where the leak is. From there its a matter of either applying more keg lube, or trying to replace a seal, repeating the process above and seeing if it fixes the problem.
If everything is ok with the keg on its own, turn on the CO2 pressure and dunk the disconnect in water to ensure all your fitting are air tight. Bubbles will be obvious under water if it is your disconnect. Last and final step is to hook up the disconnect and again cover the entire disconnect and base of the post with soapy water. If the leak is there you will see it, and if it is indeed the connection, you can attempt to fix. Sometimes an ample amount of keg lube all over the post will help, though in some rare cases I have had to replace the entire post due to some dents, dings, and chips which were not allowing the disconnect to seal properly.
I guess the key is to attack the process systematically, break down each source of the leak one at a time, and once you find the real source you can deal with fix. If you haven't figured it out from above, keg lube works wonders in terms of making proper seals. It always pays to use lube liberally