Balsamic reduction sauce?

First, I am a hack in the kitchen but really enjoy listening to those at my table moan with pleasure as they eat my food. With that said...I have a question about making a balsamic vinegar reduction sauce. Most instructions I have read tells me to reduce by half. How can you tell you are there with out measuring ? It seams to really thicken up more and more as it cools. I have tried several times and have either ended up with way too thick, caramel, or too thin. All taste pretty darn good however it is a bit difficult to get Caramel to drizzle over your salad.....please help. :)