My cat has been drinking excessively and now her litter box is so saturated within a day or two that the clumping cat litter looks like mud. She has now just started losing huge clumps of hair. I know if I take her to the vet that they will want to run lots of expensive tests which I don't have alot of money for. If someone can help me decifer if this is kidney failure it would be nice. She is over 20 years old and I unfortunately would only bring her in to have her put down if she is really sick.
The saturated litter box is a sign of Kidney disease, but also diabetes, and probably a lot of other diseases.
I think your best bet is to call the vet, explain what is happening, how much you can afford, and what would be the best interest of the cat.
But, whatever it is, your cat is sick, and sometimes, its better to let a cat go peacefully rather than run a series of tests, especially for an animal this age.
Why are you focusing on kidney failure? At age 20, she could have any number of things. Why not focus on the cat's comfort and well-being? If you can't afford lots of tests and treatments, why not have the vet give her a quick look and if he thinks she's suffering, put her to sleep.
It does sound like it according to the list below, but you should have expected it. Your baby's getting very old and kidney failure does appear when the cat is elderly, there are processes to slow down this process. If i were you I'd give my vet a call, and share your concern with him/her, they can give you better description and advice on how to check and what to do and what not to. but anyway here are the symptoms, i found remember they occur late in the day. So watch your cat closely!:
* high level of urination and drinking (supply plenty of fresh water)
* apathy (due to poisoning of the cat's body by the retention of the
waste products of metabolism – uremic poisoning)
* sluggishness (due to poisoning of the cat's body by the retention
of the waste products of metabolism)
* loss of appetite
* loss of weight
* dry coat – unkempt appearance
* tongue and gum ulcers
* brownish tongue
* ammonia smelling breath
* vomiting yellowish fluid
* diarrhea
* anemia
* bleeding of the intestine
* coma (terminal)
I do hope your cat doesn't have it though. 
Hope i helped