How to tell where rain is getting into the window frame

[ZoeG]

It's a single glazed wooden sash window south facing top floor. The underside of the sill is rotten on the inside of the building and all the plasterwork underneath is blown/bubbly/ruined but I can't tell where the water is getting in? I can get the bottom half of the frame replaced I guess but don't know if it will just rot again if water is getting in higher up?

Any advice?

Ta

[KidKruger]

Often it's where the water collects at bottom of window frame and soaks through joints where the upright sides meet the sill. Can also be coming under sill with help of wind if not properly sealed.

Is the windows' paintwork in good condition ?

[ZoeG]

Hi yes the paint work is good and I don't think the water is getting in at the base of the verticals that seems to be rot free. Interesting about the wind pushing the water up and under the sill it could be but are there some tests or something I can try to work it out!?

[KidKruger]

Hold a small hand mirror out the window and have a look at te reflection of underneath the sill.