In the case of of South Dakota v. Dole 483 U.S. 203 (1987), the Federal government was attempting to withhold federal highway funds from States that did not pass a minimum drinking age of 21.  Under the 21^st Amendment, the power to regulate alcohol fell mainly to the States.  The US Supreme Court upheld the law, and found that there were several conditions to deciding if a law met the conditions to be legal under the Spending Clause:

• The tax must be in pursuit of the general welfare.
• It must be unambiguous.  There can't be a failed understanding of what is expected of the States.
• It must be related to the Federal interest in particular national projects of programs.
• It must not violate any other Constitutional provisions.