I realise it is harder with a multi-cat household. There were 5 cats here back when Bertie was first diagnosed, and he is also an indoor/outdoor cat. It's often a matter of watching and waiting. (And if you find pee on the clingfilm and test it you'll soon know whether it is Smiffy's or not...

)
Here's a link to Ketodiastix on Amazon UK. But it should also be possible to get them from most pharmacies, so maybe a friend/neighbour could pick some up for you if you don't want to order online?
Note: 'Ketodiastix' test for both urine glucose
and ketones, so these will give you more information than 'Ketostix' that
only test for ketones.
https://www.amazon.co.uk/Keto-Diastix-Reagent-Strips-Box-50/dp/B000M2IL84/?tag=felinediabetesfdmb-20
There is more info on how to use them here:
http://www.sugarpet.net/urine.html
Testing for ketones is very important, especially with recently diagnosed cats and those who are in high numbers. Anything above a 'trace' ketone reading is a reason for a vet visit ASAP. But, hopefully, testing Smiffy's pee for ketones will just be a reassurance that all is OK.
Eliz