How do I know when the plunger has worked?

Fill a bucket full of water and start pouring it into the bowl. If the clog is gone, this should make the toilet flush and the water will go down. Be sure to turn off the water under the toilet with the knob that is on the wall there so the tank will not keep filling up. If the water goes down....the clog is gone. If it looks like it is NOT going down, the clog is still there. Then plunge it again. If this all fails...call a plumber.

If the plunger worked, the water level in the toilet will be back to 'normal'.

Even if there's still a clog and the water just seeps past it to 'normal',

the toilet is designed not to overflow if flushed when clogged at the 'normal level.

Pour a bucket of water into the toilet. If it easily goes down then try flushing it. You will have slow drain at first but it should not overflow if the blockage is clear.

when to line clears to the normal water level in the bowl it is normal === if that does not happen then you need to call the plumber ....

if the bowl didn't empty itself in a few seconds, then it's still blocked.