put another call in. if it is visibly leaking, the engineer should be able to see where it is coming from, or witness marks if it has stopped. Try not to have a late-in-the-day appointment.
Film it dripping, with your phone or a camera, especially if you can see where it is coming from.
Catch the drips in a bowl or basin, preferably glass/pyrex. He should be able to tell easily if it is condensate, clean water, or radiator water.
It may possibly damage your worktop or kitchen units, the chipboard edges will swell if it gets wet, and certain marble or stone as they are damaged or stained by acids.
Bleed a radiator onto a tissue and see if the water smells/looks the same.
Tomorrow, go outside and look at the pressure relief pipe, which goes through the wall behind the boiler. It will be a 15mm copper pipe, curved round to point back at the wall so that any water ejects squirts at the wall and not any passer by. Is there any sign it has been dripping?
Look at the ceiling above the boiler. Has water been dripping down and falling on the boiler?