It will be water from external rainfall - water from an internal leak in the house - or water from condensation.
Of course you can ask them to check it and advise - if the window has a bad seal with the frame that supports it they will be pleased to know about it and to solve it.
If you think its leaking in from outside then spray it with a hosepipe or throw a bucket of water over it - then you'll know.
Remember - you're paying the bill - the installers will get about one third of it.
Peccavi, October 2010
