In home games, I know it's up to the individuals.

However, in a tournament setting - at what point can a team be declared illegal for being over in points?

In the venues that I have played, we are told to check our opponent's team's points BEFORE the match starts. This implies that once the match has started the team cannot be disqualified for their build.

Is this a venue specific thing? Is there a proper time for this to be checked?

I feel that once you have started the game and you find your oponent has an illegal team, then you play it out. It should be up to your judge to check each team's build total and legality before the tournament begins. It doesn't hurt for you to double check it as well. Waiting until the match is over before declairing an illegal build is just as improper to me as building it. It sounds like sour grapes when you lose and gloating when you win.

L'Chayim

The judge checks them all? If there's a large number of players, that could take awhile. Of course, having a person "sign-in" their team is probably a good idea. That way the judge can make sure all the pieces are legal and that the team size is legal.

If it's before the first round, and before game begins, then the player should be given a chance to (quickly) redo their team.

If the game has already started, then the overbuilt team should have already been verified by the opponent. They should have literally signed off on the other guys team (assuming you are using the team build sheets that WK provided or something similar). If that's the case, both players should be penalized. One for overbuilding, the other for certifying the error.

Punish just the overbuilder? That's not entirely fair. The opponent may be cheating too. He could see that it's overbuilt and decide "hey, I can take the team and get all the points - if I call this guy on the build, then I'll probably get a Bye and that's 0 points. And hey, if it looks like I'm about to lose, I can suddenly realize that the math is wrong."

This is why I always announce the importance of validating your opponents team and signing off on it at the beginning of the match.

As Norm says the point of the "check your opponent's build" is to avoid that mid-round or after-the-round "hey, what's this?" moment, and if that's being done then before the round is the only right time, since that's when you find it.
If you're not doing a cross-check, and many smaller events don't, then you've got an obligation to call it immediately when you notice it. Anything else is, basically, cheating on your part.

Many years ago, when I was fairly new as a Warlord (MK Judge) I had an eight player Sealed where five players were found to have over-built, and all were I believe innocent errors. Doing the math in their heads, or with paper-and-pencil, and just getting it wrong. I carry half a dozen spare calculators now.