How long does a fruit and sugar mixture last? I made a sauce with frozen rhubarb and raspberry mixed with sugar and reduced it until there is hardly any juice left. I put the leftovers in a sealed container in the fridge. Would this still be good? Is there anyway to tell when this is going bad?
It depends on a lot of factors: what you're describing isn't much different from making jam, and homemade jam is good for about a month, once you open it. Assuming you got the temp high enough to kill all the bacteria, the time will depend entirely on PH and water content. The more acidic, and the less watery the better.

Generally things like this freeze well though, so, when in doubt, throw it in the freezer.