The closest you can get a free esn check is to call Verizon and ask them if those esns are clean or not. But even if they are, if they are Verizon prepaid esns at all, forget it.
Theres a way to check Verizon esns online too.
It does work (for Verizon) but it won't tell you if it is a banned prepaid Verizon device (I just tested it with a Verizon prepaid VX5500.
One clue is that a Verizon prepaid phone lists the model as a "pp" type (prepaid NOT page plus) after the model number inside the battery compartment. The VX5500PP for example. I do not know if this is universal on all prepaid Verizon models however.