There is typically silicone that runs around the seam (where the wheels are bolted together). This is on the inside and the tire will have to be removed to see it. Sometimes this get damaged when a tire is being mounted. It is very easy to apply a new bead of silicone to fix the leak. The best thing to do is submerge the wheel (with tire) and see where the leak is. If nothing else you can put the wheel in a bath tub. If the leak is at the seam where the wheels are bolted together, the you need to do the above.