I can't remember how the JV3 does it but in the service menu just go to the [HASHTAG]#Test[/HASHTAG] menu and find the Vacuum test. I think it is under the Action Test menu but it's been so long since I worked on one of those I can't remember. An easier way is just look at the vinyl; when a print starts. Usually there are some small buckles in the vinyl but when the print starts, the vacuum turns on and you can see the buckles get sucked down.