Electric heaters are 100% efficient. You can measure their consumption if you can plug them into a power measurement device like this KillAWatt meter.
Kill A Watt from iKitchen.com
Once you know the current draw, you can convert this into BTU output, if that is what you want. You will also know how much electricity they use, which will tell you how much they cost to operate.
If you can't plug them into a measurement device, you can turn off all the electrical consumers in the house except the heaters and observe how much the electric meter advances in an hour.