How can I tell if my car is misfiring?

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I posted here a couple of months ago that my car judders back and forth when driving...this happens randomly, sometimes not at all if I drive and it can happen when I drive later during the day.

Some users here suggested it may be an engine mount, the garages don't seem to notice anything wrong. So is it possible it may be misfiring or something along those lines?

Here are the symptoms of my car:

- Judders back and forth when driving and stationary
- Opening bonnet I can see the rocker cover box vibrates now and again which causes the car to judder
- Opening the gas value under the bonnet...the engine moved towards me then back to its normal position once I put back the gas valve.
- The rev meter works fine.
- When the car is stationary I can feel the car vibrating very slighhtly.
- When driving at high speed the seems to be little or no judder, the car judders more at lower speeds.

Any other things I should check would be helpful, thanks. So is this a misfire, mount or something else?