Every motherboard that has at least an AGP 4X slot is 1.5V.
If your CPU is an Athlon XP, Athlon64, or Pentium 4 and it has an AGP slot (versus a PCI-express slot) then it's either AGP 8X or AGP4X, which is 1.5V like all AGP 4x/8x cards. You should be fine unless you're running it with a Pentium 2.