Yes, it does check.
Pokemon with the following ribbons are also unable to get through to the GTS:
- Classic Ribbon
- Premier Ribbon
- Birthday Ribbon
Basically, the GTS will go to a high extent to check if it's hacked or not. So, if an event Pokemon (like Jirachi) were only ever released with an Event ribbon of some sort, and a Jirachi without this ribbon attempted to go into the GTS then the GTS would detect it's been hacked and will not let it through.
All these Pokemon go through Nintendo's servers to make sure it isn't hacked.
Hope I helped. :)
Source and Experience