Thanks for your question.
First, try filling it as much as possible with air, without risking the mattress bursting.
Examine the mattress. You might see or hear the leak. Mark the leak if found.
If that doesn't work, add a small amount of liquid dish soap to a spray bottle of warm water and begin spraying the mattress in small sections. If you see soap bubbling, that is where the leak is. A soapy wet rag wiped on the mattress will work also. Dry the part off with a dry rag and mark these leaks with a marker so you can patch the hole later.
If that doesn't work and if possible, put the mattress into a bathtub or pool with water and dunk parts or the whole mattress until you see bubbles rising regularly from the mattress. This will indicate leaks. Dry the part off with a dry rag and mark these leaks with a marker so you can patch the hole later.
Also, you can try moving your hand slowly along the surface of the mattress. Often, you may feel the escaping air against your skin. Or move your head along the surface with your ear near the mattress. You may hear escaping air.
Pay close attention to all seams. These are common places for weak points to occur which can develop leaks.
You can also try moving the mattress outdoors and use a hose to cover the surface of the mattress with a small stream of water, looking for bubbles where the water runs on the mattress surface.
Also, be sure to check the area around the air valve as this is a common place for leaks.
Hope this helps,