There's a simple way to check for leaking tires when you see that one might be losing air. Use a bottle of spray cleaner or make a mild soap solution and put it in a spray bottle. Either one will do the trick, just spray it on an inflated tire that's still mounted to the vehicle and look for bubbles that indicate the source of a leak.
Make sure to operate it in the following steps.
1. on the surface of the tread
2. around the valve stem
3. inside the valve stem (after removing the cap)
4. where the tire seals against the metal wheel (the bead)
5. across the metal wheel surface
Take your time and look carefully. Bubbles can appear as very fine foam that slowly exhibits itself. If your tire pressure is low, then pump it up a bit to help make the bubbles more apparent. I often make a habit of spraying twice on the same surface just to make certain I've covered the area well.
Rotate the tire to check the remainder of the tread that is on or near the ground. It's likely that your source of leak will be found on the tread as this is exposed to puncture hazards like nails and screws. Valve stems are also troublesome because they can deteriorate and the valve core inside the stem can also lose its seal.
A failed bead and cracked metal wheel can be a cause of leaking tires, but it's rare. To check for this, be sure to spray the outside and inside of the wheel and bead.
If you locate a foreign object in your tread that is causing the leak, it's best to leave it in place until you're ready to fix the leak yourself or take it to a shop for repair.