It may be that you have recently moved into a property in London that already has Double Glazed Windows fitted and you want to know if they are going to last for a few years yet. If so, there are a couple of quick signs to look for that could indicate trouble in the near future.
Firstly check the exterior of the uPVC frames for signs of distortion, cracking and severe fading; next check the rubber gaskets on the window beads for the same symptoms. If the vinyl and the seals are starting to perish, then if it’s possible to repair them do so as soon as possible.
Secondly, check the mastic sealant around the frame for deterioration, if it is just this which is on the way out, then it can be repaired without too much drama. Inside, you should open and close each window to check for tight fit and proper function of hinges & locks, locks can be replaced and so can hinges, but if the frames are cracked at the corners or warped they may be irreparable.
Make a good check for any signs of water penetration & damp around the frames, longer term damp issues would show up in the wall, with water marks, peeling paint or wallpaper and plaster.
Any sign of water condensation inside the sealed units spells big trouble.