Separation logic is a popular approach for specifying properties of recursive mutable data structures. Several existing systems verify a subclass of separation logic specifications using static analysis techniques. Checking data structure specifications during program execution is an alternative to static verification: it can enforce the sophisticated specifications for which static verification fails, and it can help debug incorrect specifications and code by detecting concrete counterexamples to their validity. This paper presents Separation Logic Invariant ChecKer (SLICK), a runtime checker for separation logic specifications. We show that, although the recursive style of separation logic predicates is well suited for runtime execution, the implicit footprint and existential quantification make efficient runtime checking challenging. To address these challenges we introduce a coloring technique for efficiently checking method footprints and describe techniques for inferring values of existentially quantified variables. We have implemented our runtime checker in the context of a tool for enforcing specifications of Java programs. Our experience suggests that our runtime checker is a useful companion to a static verifier for separation logic specifications.