If the play is an older one, ask at the Reference Desk. There are a number of sources of reviews, but the one to use depends on the play in question. For example, American Drama Criticism (Ref PS 332 .E33) indexes reviews of original productions in the 20th century. If the play ran in New York City, use the periodical National Theatre Critics' Reviews (formerly New York Theatre Critics' Reviews, 1940-1996), which reprints reviews from major New York media. (Shelved with King periodicals.)
Be sure to ask at the Reference Desk if these books do not help. There are hundreds of theater and literature reference books and other important sources of information.