dipstick, oil pressure light and temp guage all there to help you monitor it, but if it's a drip every fifteen minutes sat on the drive it's likely to be more when hot and running. Usually when you are dropping oil you have a spotted tailgate, if you can't see out of the rear window then stop

, sorry but again you can use a judgement on how much the loss is by how filthy the rear of the van is after x miles. How long since oil change ? You should see the evidence on there.
So what oil filter do JK sell ? If you haven't got the box anymore it should be on the filter and still readable now.