To prevent the president from ideologically slanting the judiciary, what powers does Congress have?

Congress has the power to advise and consent on judicial nominees.

This means that Congress has the power to approve or reject Presidential nominees to the judiciary.

The effect of this is that if there is an opposition Congress (Congress from a different party) then Congress has the ability to shape the type of nominees they are getting from the President as nominees that are too far afield from Congress will not be approved.

If Congress aligns with the President's ideology, then there is no check.