Final answer:
The most effective way for Congress to check a president's foreign policy is by cutting off budget support for the policy, directly influencing its implementation.
Step-by-step explanation:
The most effective way for Congress to prevent a president's foreign policy is through various checks and balances afforded to it by the U.S. Constitution. One of the most impactful measures Congress can take is to cut off budget support for that policy. This employs Congress's power of the purse to directly impact the implementation of a foreign policy initiative. Unlike simply passing a resolution, which is largely symbolic, cutting off funding has tangible effects on the president's ability to carry out foreign policy. Refusing to ratify a treaty is also effective but applies only to formal treaties, not executive agreements, which do not require Senate ratification. Other options, like declaring an executive agreement unconstitutional or rallying public opinion, either require judicial intervention or are more indirect and less certain in their impact.