Mathematics can serve many functions in physics.  It can provide a computational system, reflect a physical idea, conveniently encode a rule, and so forth.  A physics student thus has many different options for using mathematics in his physics problem solving.  We present a short example from the problem solving work of upper level physics students and use it to illustrate the epistemic framing process:  "framing" because these students are focusing on a subset of their total math knowledge, "epistemic" because their choice of subset relates to what they see (at that particular time) as the nature of the math knowledge in play.  We illustrate how looking for students' warrants, the often unspoken reasons they think their evidence supports their mathematical claims, serves as a window to their epistemic framing.  These warrants provide a powerful, concise piece of evidence of these students' epistemic framing.