Soapy water sprayed or splashed on will show where a leak is as you can see the air coming out through the foam etc. I usually just grab the foam off the top of the soapy water and apply it by hand. It's an old trick to test for gas leaks. For example if you connect a new gas bottle to a barbecue you can do this to make sure no gas is escaping. You'll see a leak fairly easily. I first saw this when a gas installer was changing a meter more than 50 years ago.
Martin

