I would just take it to a Verizon store and see. It isn't as easy as "GSM v. CDMA," since the phone has to be compatible with both the air interfaces (not just GSM/CDMA, but GSM, UMTS, HSPA, HSPA+, 1xRTT, EvDO, and LTE) and the radio bands the carrier uses. In the US, there are eight radio bands commonly used--one is specific to Verizon and two are specific to Sprint. Depending on how customized the phone was for the carrier, it may or may not work, and even if there is minimal compatibility, you may not have access to the full array of Verizon services (Verizon's main LTE deployment is in the Upper 700 MHz band, a band that only Verizon uses). Best to ask.