I find myself pondering this very question while attempting to eat three small oranges that are probably about two months old.
So, Question: how do you know if an orange has gone bad?
Answer 1: If the color of the orange is anything but orange. There's a reason these fruits are not called "browns" or "slightly burnt yellowishes"
Answer 2: If your dog loves oranges and then when presented with said fruit is then profoundly perplexed. My dog loves oranges, but he sure wasn't going for this one. I let him sniff it and he sneezed three times in a row, looked at me with a "what the hell is that" expression, then went to his bed. Trust the instincts of wildlife, if they're not eating - then maybe you shouldn't either.
Answer 3: If peeling the orange burns your fingers, this could be a sign. Ever stuck your finger in a pencil sharpner? No? Well either way, you get the mental picture.
Answer 4: If the orange tastes like a screwdriver. I always wondered why screwdrivers were such easy drinks to make. Well now you can make one without even visiting the liquor store. Bon appetite. Also kills germs that cause bad breath... think of it as a self contained 90 proof fun ball that has all the fighting power of Listerine in every bite.
Answer 5: If after consuming the said "fruit", you are then overpowered by a strong sensation of non-reality. I'm not sure if I was a famous person in a previous life, but now I am pondering this very question. Imagine Alice in Wonderland or Neo in The Matrix, this is the same small little pill that can alter reality. If Disney "Fantasia-esque" weirdness occurs, then count your fruit as one of the possible rotten ones.
In with the new
1 year ago