Before you put beer in that keg, give it a good cleaning inside and out, then sanitize it. And while you have some sanitizer still in the keg, seal it up, put just a little gas pressure on the In port (like 1 PSI) and carefully run some sanitizer from the Out port (just press the poppet in with something that won't scratch up the post).
Finally, with some sanitizer still remaining in the sealed keg, raise the gas pressure to ~15 psi, and check the posts, poppets, lid and relief valve for leaks. You can use a dish soap/water mix, or a standard Star San solution, with either a soft brush or a spray bottle, and look for bubbles. If the lid leaks, try raising the gas pressure to 30 psi for a few seconds to see if that seats the lid seal, before dropping the pressure back down to see if the seal holds.
It's best to expose any problems now, before you load the keg with beer.
fwiw, when I get a new-to-me keg, when I clean the keg inside and out I also take the posts off, pull the dip tubes, strip off all the original O-rings, take out the poppets, clean the tubes and posts using a dip tube brush, before reassembling the keg with new O-rings. Then I do the sanitizer and leak-test thing...
Cheers!