if you have a bottle of liquid saying it has however many carbs per 100g. but the bottle is measure in ml's. how do you work this out without weighing it?

If it is a water based liquid you can assume that 1 ml = 1 g. It won't be exactly that, but it will be very close -- as close as you can get with any home measuring device.

It is weird they don't tell you that!

so vinegar is going to be about that to?

Yep! It's about 95% water and 5% acetic acid.

does balsamic vinegar have some kind of sugar in? its so high inc arbs

Yes - it is the one vinegar that Dr Atkins in DANDR told us not to use because it is high in sugar.