The War Powers Act does provide a check against presidential overreach, but it is not an especially strong one.
No President has ever acknowledged that the War Powers Act is actually constitutional. Therefore, it is not at all clear that the act could ever be used to force an end to a military action taken by the president. This weakens the effectiveness of the act significantly.
However, the act does provide a check. It forces the President to at least justify his (or someday her) actions to the people. This allows public opinion to have a great effect on whether the President's actions are allowed to stand.
So, the War Powers Act provides a check because it forces the President to expose his actions to somewhat of a public referendum. It does so by making the President at least consult Congress over his actions.