Use a spray bottle with a little dish soap and water..air it up..you can safely go quite a bit above operating pressure,but 10 psi or so will be plenty to give you ample time to hunt for it..spray it down all over..including the valve stem and beads and look for the bubbles..
If it is in the tread area plug it..plug kits are cheap and easy..just follow the directions..
If it is in the bead we can tell you how to resolve that also..it isn't that hard..but I don't want to just give you a one line instruction for safelty reasons..