Think of it when you see pancytopenia (or bicytopenia) and splenomegaly without peripheral lymphadenopathy. From a recent review:
The diagnosis of HCL is not usually difficult, given its characteristic clinical, morphologic, and immunophenotypic features. Most patients have splenomegaly in the absence of peripheral lymphadenopathy, and a variable degree of cytopenias, including monocytopenia. Typical hairy cells are evident in the peripheral blood on a routine smear (or on a buffy coat preparation) in more than 90% of cases.