Spot test, like everyone's saying. Usually reactions will show up immediately, but I've also heard that it may take an hour or two, depending on the toy and the lube.
If you want to be super-duper safe, spot test for every new bottle of lube, even if it's one you've used before, because the precise ratio of ingredients in each batch can vary a little bit. Usually it won't be enough to make a difference, but it's another precaution to consider, at any rate.
Wet Synergy is a hybrid lube, so will it harm my silicone toy due to it being of both water and silicone base?
I briefly used Wet Synergy the other day for the first time with my Lelo Gigi and noticed when I was cleaning it, it seemed as if a layer of the silicone was peeling off! I figured since it was a hybrid it'd be fine but I may have jumped to conclusions too quickly! I haven't tried it again in fear of ruining it but I am going to do a spot test and leave it over night and see what happens. I'm crossing my fingers that it was only a flukey thing since I love this lube for almost everything.
If you look at the ingredients though, there's a lot of silicone in it. It's basically water, silicone, a polymer, some more silicone, some preservatives, then some more silicone. All that silicone is what makes this lube so amazing (in my opinion) but that also means it can potentially mess with your silicone toys. There are other hybrid options that are fine for silicone toys (Liquid Silk being one of them), but this one is full of silicone.
Do a spot test though! Seriously! I'd recommend leaving the spot test overnight. Like everyone else said, it should be relatively instantaneous, especially if it's a lower quality silicone, but, leave it over night. Sometimes it can take a while depending on the quality.