How to tell if a Pokemon Page is updated for the Stat Rebalance?

I believe all of them on GP are updated to reflect the changes with the CP rebalance.

One test would be to ask the site what the CP of a 15/15/15 Espeon at level 40 is. If you get 3000 back, it has not been updated. If you get 3170, it has been updated.

Alternatively, use Aggron, a value of 3000 means it is updated, less it hasn't.

Not sure what to do if the site only gives IVs for values, perhaps know the new and old value of one of your pokemon, and see what it tells you.

Gamepress's stuff was updated the same day (probably the same minute) the CP changes went into effect.

If you mean CP and stats, likely as soon as the rebalance happened, as for their bios/overviews, most of the meta irrelevant seem to have gone unupdated since release or even the original gym system in a few cases

I could be wrong but I think they did the conversions the first time we had the rebalance, when it lasted all of 20 minutes. They likely just flipped a switch when they released it for real.