how can you tell fresh ricotta is going bad - is it the pink cast?

Even before fresh ricotto is so far gone that it's pink, it just doesn't taste as wonderful once it's just a few days old. I always try to use it up pretty quickly.

Here's an informative link from the Mayo clinic about the safety of mold on cheese:
http://www.mayoclinic.com/health/food-and-nutrition/AN01024

P.S. Throw out the ricotta.

The pink stuff is mold. Get rid of it. The ricotta and the mold, that is.

I'd hazard a guess that the pink is bacterial growth, and I'd toss it. Some cheeses are fine, even if mold is growing on them--you just cut the mold away. But those are hard cheeses--soft ones like ricotta should be discarded if you see any sign of mold (or bacteria) growth

If your fresh ricotta has any pink cast to it, I'd say it's already gone. I just look to see if the ricotta's still pure (relatively) white, no spots or anything. I give it a sniff and if it has a really strong odor, it's probably gone bad.