You can get a uv light from Unger for twenty quid do it at night .main prob with cat pee is it always Tom that marks it post , thus spraying up was door frames ,skirting , so most of the time it's these areas you need check ,should have said the UVA light shows up urine stain up as white drop lets ,just. Clean al these areas with bleach don't get any on carpets though , it's very important you check these areas this is often left ,and you get calls to reclean the carpet , when its actually the spray up the walls etc,urine if strong will ,and does leave stain on carpets ,some times you improve it but never promise that you can remove it I've never never found anything to remove yellowing ,specialy old stains