Using an Induction Heater to melt multiple types of metal
You seem to be asking how to measure the frequency once you've already gotten it oscillating. That's much easier than getting the circuit oscillating properly in the first place, but so be it.  I would recommend a small pickup coil a safe distance from your heating coil.  Send it to your counter or oscilloscope and you should see a nice signal.  Now for dimensions, I'd take a guess that at 1 MHz and with a 5 kW heating power you'd probably want a coil area of about 1 cm squared and about 10 turns.  Insulate it so you can't burn out your counter/scope if something goes wrong.