Bloods, scan, sedation (try scanning a cat without!), overnight care.
I think too many people put themselves in their cat's place when it comes to a) treatment and b) PTS. Cats live for the moment, and don't understand why we are putting them through the things we do. Equally if they do need to be PTS they don't know in advance that they will be dead, that isn't even a concept for them - they just go to sleep.
There's also the thought about just how much we are prepared to make our pets go through for what result. My Boycat would be a perfect example. He developed very fast progressing CKD when he was only young and we knew he only had a short amount of time (in the end, 6 months from him starting to show any signs to having to be PTS). He was an exceptionally nervous cat around anyone except me, exceptionally thick too (we think he was either ND or had had a brain injury when a young feral). Vet and I went through all the options at each stage then we got to the time when he was getting dehydrated despite the amount he was drinking. The options then were basically that a visit to the vet every week for sub-cutaneous fluids would help extend his live by up to maybe a few months at most. This would distress him so much we both agreed we'd just let things take their course and PTS when his quality of life deteriorated a little more.
I also now have Tobias, who was a stray for years. Again very little trust of almost all humans, can't be picked up etc., has to be sedated for us to even get him to the vets. Gets separation anxiety if shut away from Girlcat for any reason e.g. starvation the night before bloods or if he needs full anaesthetic the next day. He has stomach issues, and in the course of getting the reason for that diagnosed we discovered he has FIV.
He has thickening of his lower bowel walls, and beyond the scan by the specialist vet sonographer the next thing would be surgery which is a big no no for a cat with FIV except in life threatening conditions due to the chances of infection. The very action of taking him to the vet, an overnight stay away from me & Girlcat, a major operation the next day & a further 24 hours away from home and us with strange people around would be so very distressing for him and would probably undo most of the 3 years I've spent turning him into a happy cat. There's a 50/50 chance it could be a cancer and the prognosis for that with surgery and chemo for even a healthy cat is maybe a couple of years.
So Tobias will be monitored, loved, cared for as a happy cat then PTS when his quality of life deteriorates. I will be heartbroken, as I was when my soulcat Boycat was PTS, but content in the knowledge that he had his best life with me then one day just went to sleep.
TL:DR - cats don't understand the concept of dying, or understand what we do to them to help heal them. Don't anthropomorphise them.