Lumpy after startup

Hi All

When I 1st start the yeti it runs lumpy with the rev counter bobbing up and down and extra notch for around 10 seconds, after this it's as sweat as a nut all day.  It's had a new battery as it was originally slow to turn over so wondering if this could be a bad glow plug? Guessing after it's got a quick bit of heat it's running fine

So question is how does one test the glow plugs with a multi meter and what should I be looking for as never had to get involved before.