How do I find a leakage in an electric pump built-in on an air mattress?

Fully inflate the mattress. Bundle the cord in a tight, flat package with rubber bands and tape in place over the connection or stow it away in the recession provided. Use lightweight Saran Wrap and typical desk tape to install an airtight patch over the entire area. The plastic wrap bubble will be loose and it will be possible to press out excess air before putting on the last section of tape. Be sure to lightly burnish the tape with a fingernail to close any gaps. Any leak will at least fill out the looseness or slack in the plastic patch.