Set the multimeter to the ohms setting (Ω) - there are sometimes different ranges on a meter, sometimes listed as “200, 2K, 20K, 200K, 2M, 20M”. Try it at the most sensitive (20M, in the example).
If there is just one setting, then it will probably autorange, and give you an M or K in the display, as appropriate. A reading of “0L” on some meters means infinite, or open circuit; a reading of 0.00 means continuity, or closed circuit.
Test with the probes not touching, then touch them together, just to be sure it’s working.
With the machine unplugged, and the element power turned on, try measuring with one probe on the boiler and the other probe on one flat blade of the power cord. Then try between the boiler and the other flat blade. Try between each flat blade and the ground pin (if there is one). It should only show infinity (0L on some meters).