How long can you store sugar syrup? Recently, we made a variety of cocktails from Ottolenghi and Scully's NOPI book. Now, we are left with two sugar syrups. According to the book, they can be stored a few weeks in the fridge. Skip ahead a month or two, and we're now left with two bottles of syrup that I'm not sure whether they are still safe to consume. As far as I'm concerned, the sugar acts as a preservative (like in jams) and it should be no problem; however, I'm a bit reluctant to ignore the advice of a distinguished chef - but perhaps he was just referring a change in flavor.

The syrups in question are 50/50 water and sugar (in weight), infused with cloves or saffron, each boiled for about 10 minutes. There is also a recipe with fresh ginger that is not cooked but blitzed, which may be more of a problem than the others.

How long can a syrup be preserved? What, if any, are the signs of a spoiled syrup?
From my experience, sugar syrups don't last too long (a month tops in the refrigerator). You may be able to prolong their shelf life by adding something acidic, or anti-microbial (I think some spices may work) but not by much. As for jams, I don't think it's the sugar that preserves them so much as the acidity or heat from canning/preserving. 

Sucrose is actually something I've used in a laboratory to culture microbes. Many, including a lot of ones we're familiar with contributing to rotting foods in our kitchen, can use sucrose as an energy source. 

That being said, if it passes the smell test, doesn't look cloudy (more than it initially was) it's worth trying a little if you're a home cook. I'd say the cloudiness is the best indicator, especially if it looks like there are strands/strings of cloudiness in the liquid.