I'm sorry to hear about Fifi. I have had several pigs with developing kidney failure (often from stones but some for other unknown reasons although lymphoma was suspected in one) Kidney failure can be a result of old-age organ failure, infection, malignancy or stones. It is possible for a good vet to diagnose kidney issues just from palpation. I know of two vets who are able to do so and they have determined correctly on several occasions (without knowing in advance any blood test or Xray results I have previously had done) whether my pigs have enlarged, shrivelled or indeed normal kidneys.
As to follow-up tests: Diagnostics requiring a GA can be risky at the best of times and even more so with a piggie with kidney failure which will also impact on heart function. An Xray may or may not show the actual kidneys but is useful to determine if the kidneys contain stones as the reason for their enlargement as it's the stones that would be seen on Xray. An ultrasound would be better (and could be done without anaesthetic) if they are looking at imaging the kidneys themselves and measuring size/looking for tumours. However most vets would resort to bloods to test for kidney function when kidney failure is suspected ....and having had the discussion recently with my own vet regarding one of my pigs, they said that even the bloods might still show as normal if one kidney had failed and the other was still working. Getting blood from piggies is extremely risky as it has to be taken from the main vein close to the heart and even my really experienced exotics vet has, on occasion, bottled out if he can't get it right at the first attempt. I will only let them take bloods as a last resort.
I'm afraid that nothing can really be done for piggies with renal failure apart from palliative care - although it is good that your vet has put Fifi on meds to slow down the progress of the disease. It is indeed benazepril and it works by preventing constriction of blood vessels thus reducing blood pressure in the heart and in the kidney filtration system so thus reducing the strain on both organs.
Be on the alert for signs of pain (we have previously used high dose metacam but now prefer to opt for tramadol for pain relief) In your case metacam may be contraindicated (ie best not to use) with the heart meds Fifi is on. If you get bloody wee or Fifi becomes lethargic then you may need to make an emergency trip to the vets.
I hope she will enjoy many more piggie years before it is her time.
Best wishes
x