For most insulins they are 100 units per ml (there are stronger ones though) - anything that says it is U100 strength means 100 units per ml (U200 would mean it had 200 units per ml)
If it is a syringe for insulin then that syringe can hold 100 units of insulin which for most insulins would be 1ml (IU's are different amounts for different substances I think)
The simplest way would be to fill the syringe, then empty it into a small measuring vessel and see how many ml it is, however, why do you need to know? I'm a bit concerned you might be using something not meant for what you want to do.
You'd need a very, very small measuring vessel to be able to measure 1 ml. Syringes are perfect measuring vessels to measure very small amounts, like cat medication or the stuff you use to kill nettles with.
You'd need a very, very small measuring vessel to be able to measure 1 ml. Syringes are perfect measuring vessels to measure very small amounts, like cat medication or the stuff you use to kill nettles with.
You can get some tiny pipettes on eBay (says the voice of experience, because I use them to measure fertiliser and chemicals for my aquariums).
But I don't think they would be anywhere near accurate enough for assessing precise insulin quantities.