Sometimes you don't have a place to submerge your pad to find leaks. In that case there is another solution, albeit slightly less ideal than dunking.
- Fill a squirt bottle, spray bottle, hydration bladder (something with a nozzle to control the flow) with water near to the top.
- Add several drops of liquid soap (dish detergent works best).
- Wet your fully inflated sleeping pad with the mixture.
- Look for bubbles forming to isolate the leak(s).
- Carefully mark the location of the leak with a needle/pin (you've got to fix the leak anyway) as you will lose it as soon as you rinse the soapy water away. You may try a grease pencil or china marker but not much else will write on a soapy wet surface.
- Rinse the area to be patched of the soapy water.
- Let the pad dry and follow your procedure for repairing the leak!
I've had to use this method several times and it always works. Be systematic about it so you can keep track of what you've checked. Usually leaks occur on the bottom of your pad, or they could be at a failing seam, so target those areas first. Also you might have to refill the bottle once or twice before you find the leak, depending on the size of the pad and just how lucky you get. If you're mix isn't sudsy enough, add more soap.