The supreme Court uses one of three levels of review to analyze whether a government action in constitutional under the equal protection guarantees of the the 5th and 14th Amendment. The 14th Amendment equal protection clause (which applies to the states), and the 5th Amendment equal protection (which applies to the federal government), basically state that government may not deny a person "equal protection under the laws."
Other than strict scrutiny, the other types of scruitiny the Supreme Court uses to decide on cases involving equal protection are intermediate scrutiny and rational basis.
Strict Scruitiny: This test is used in cases involving either the burdening of fundamental rights or when government action is taken based on classification based on suspect classes (race, religion, national origin, or (sometimes) alienage. To pass strict scrutiny the government action must serve a compelling state interest, be narrowly tailored to serve that interest, and be the least restrictive means to meet that purpose.
Intermediate Scrutiny: This test is used in cases involving goverment action based on the quasi-suspect classes: gender or illegitimacy. Under intermediate scrutiny, the government action "serves an important state interest and that the classification is at least substantially related to serving that interest."
Rational Basis: This test is used when the goverment action does not fall under either strict scrutiny or intermediate scrutiny. With Rational basis, the government action need only be "rationally related to serving a legitimate state interest."