Four part spending power test from South Dakota v. Dole at pp. 162-63:

(1) the exercise of the spending power must be in pursuit of the general welfare (courts generally defer to Congress in defining general welfare);
(2) the condition needs to be unambiguously stated so that the states can make a knowing choice;
(3) there needs to exist some relationship between the purpose for which the federal funds are being spent and the condition the states must comply with in order to receive those funds; and
(4) the condition may not induce the states to engage in activities that would themselves be unconstitutional under other provisions of the Constitution and, therefore, be independently barred.

The Court also suggests a fifth requirement: that the financial inducement offered by Congress may not be so coercive that it no longer amounts to just pressure to accept the funding, but amounts to compulsion so that the states no longer have a real choice to reject the federal funds.