Get the tank refilled.
Make a nice soapy solution (or use Star San or buy the leak detector stuff at your supply store)
Swab everything with the solution and fire up the gas.
By everything I mean:
any junction (regulator -> tank, valve -> tank, hose->regulator, hose->gas fitting, slather the whole gas fitting, the post and base of the gas and liquid posts, the lid junction, the relief valve)
Start with the pressure low and watch. Keep a wrench and screw driver handy to tighten everything.
If the leak is in the lid, crank the pressure to 15 psi and see if it stops. If it does, consider getting fat o-rings.
Lastly, no leak? disconnect the gas and watch the posts on the newly charged keg.