It should have wattage printed on it. You need a short cord with individual conductors. I use a flat air conditioner extension cord with hot conductor separated with razor knife.
Check with an ammeter. Amps X volts = Watts A watthour is 3.4 BTU per hour.
A BTU is defined as heat needed to raise 1 LB of water 1 degree F from 70 Degrees F.
A lot of calculating there, and that's before we figure how much energy needed to heat the iron, or how much is lost to the atmosphere.
It is unlikely it is thermostatically controlled, so plug it in, give it an hour or two, see if it gets warm.
As an electrician, I get calls often about block heaters tripping GFCIs 90% of these problems are solved with a good extension cord.