dont you ever think when you get to a house....there still spotless!! oh well better do them anyway when we did trad they were filthy after a month.....dont know if this is a good thing or bad...
Check for bird poo then just card them if they're not in
You can look from the outside on some windows and they will look clean, look from the inside and they won't be. But of course you can't always get access to the inside. The idea of window cleaning is to keep the windows as clean as possible, not to wait until they are minging