Depends if they are painted or varnished and if the glass has been bedded in putty, varnished I just clean as I would with upvc
as they rarely cause trouble, with painted rub your finger on the paintwork to see how much the paint has degraded and if
bad give all the frames a good scrub first and go back and do the glass, if they have putty (older and listed window frames ) then do the same as painted but don't be supprised if they still don't come up well.