Coolant Gauge Inop

[Fancy_Footwork]

I've got a 95 240sx, with what I believe is an s13 dohc in it and the gauge isn't reading coolant temp. The sensor has been replaced already. Does anyone know how to test the sending unit before I waste the money replacing it? Or at least a way to rule out the sending unit? That and if I do have to replace it, is there any difference between s13 and s14 dohcs as far as that sending unit is concerned?

Any help would be appreciated