How to identify a leak

[aroundM21]

So regular double-glazed bay bedroom window. Certainly over 10 years old.

It's leaking. Even allowing for condensation, we get a lot of water on the bottom sill.

Any tricks to identify the source would be gratefully received so I can decide what to do. Clearly it could be between wall & window frame; or frame & opening windows, or in the panes ie PVC/glass. (The seals all seem fine by the way - all clear).

Please don't suggest pouring coloured liquid in as it is on the first floor ! That's what I would do if ground floor.

Here's hoping for a trick of the trade. Thanks in advance.

 

[pinenot]

Hose window from outside whilst someone check the inside...seemples

 

[foxhole]

Any trickle vents in frames?

 

[s60tsy]

if the drain holes inside the frame are blocked water can backtrack from allsorts of places, if the cill isnt level ie: runs back, it can send water towards the internal cill. simply if the sealant has given up outside, rainwater can find its way inside. etc. etc.