There are a number of informal checks and balances that Congress (or, more precisely, members of Congress) can use to check the executive. Let us look at two of these.
First, Congress can check the executive through threats to use its formal powers. Congress, of course, has the power to pass laws or to refuse to do so. Therefore, Congress can check the executive by warning that any attempt by the executive to pass a certain law or type of law will be strongly resisted. We can argue that this is happening today as members of Congress warn President Obama against trying for any serious gun control legislation.
Second, the Congress can appeal to the people. In this instance, Congress is trying to stir up public opinion to support its point of view. This can check the executive by making it worry that public opinion will go against any efforts it makes in a certain area. This has been seen recently as Congressional leaders have tried to persuade the people to oppose any deals on the “fiscal cliff” that would involve raising taxes.
In these ways, the Congress can try to check the executive through informal means.