You wait til they're carbed and conditioned. The 3 weeks at 70 we mention on here is the minimum for most average gravity 12 ounce beers, higher gav beers may take months to carb and condition. Large bottles take longer than 12 ouncers.
You take one from each case, chill them for a minimum of 24 hours, then taste them. If both beers are carbed, then more than likely all the beers are carbed, if only one is, the beer is more than likely not carbed yet and needs more time.
If neither of them are carbed yet, it needs more time...that's it. Carbing is not mystical or magical or complicated, it's yeast, eating sugar and farting co2 for a few weeks...and it's fullproof. But it's a game of patience.