Hello! Your vet can check for protein in the urine easily enough, by using a protein dipstick on a fresh sample. It can be helpful to pick up warning signs of other diseases too such as diabetes and (with a refractometer) kidney disease, so your vet may be happy to do this for your old cat if you can provide a fresh sample. I have never heard of bubbles in urine as a warning sign of protein, but a urine test may be a useful way in which to find out. If your vet knows something I don't know about this, please do let me know!
Was just something I had heard and looked it up. I know of course you can’t believe everything you read, so that’s why I asked. Thank you so much for your reply!!