Current draw can be measured with a multi-meter anywhere in series with the load (in this case, the heater). You can make a simple cable that's useful for measuring the draw of any AC appliance from an old extension cord . Cut one of the cord conductors and install a pair of insulated end connectors that will plug into your meter. The standard male bullet connector fits most meters. A word of caution: label the cord and keep it in your toolbox so that someone doesn't try to use it as a normal extension cord.
For more safety, I installed a current transformer in place of the bullet connectors. Either way, you have a handy little device with which you can run around the house measuring the power draw of all your appliances, as well as for basic troubleshooting.