That’s interesting! I wasn’t aware we had shops that could create variants on the fly and add them to cart immediately without encountering a 422 error. Typically there’s a delay of one or more seconds. What kind of mutation are you using to make them?
Regarding if there is a way to check whether a product is ready to be added to cart, there isn’t one at this time. Only way to confirm is a successful (non-422 response) add to cart request.