There's no way to be sure; this Arceus could very well be completely legit. At this point, the only red flag I can think of would be checking its IVs; if they're perfect, it's absolutely hacked. Even if they aren't I would still say the chance of it being hacked is higher than the chance of it being legit since it comes from an event that is several years old that was only available in Japan, and it is relatively common for people to hack in several boxes of shinies and Wonder Trade them (in general, it's best to be suspicious of any shiny you receive over Wonder Trade for this reason).
TL;DR It's impossible to be 100% certain, but I'd be very suspicious of any shiny Arceus on the Wonder Trade.