How do I know when my sauce is reduced enough? I followed this recipe over the weekend to try out something new:
Eric Ripert's Red Snapper and Morels in Port Reduction
My biggest issue was the sauce. It was very liquidy. I don't think it reduced properly and perhaps not enough butter? But I'm really not sure.
All the recipe says is:

Bring Port to a boil in a small heavy saucepan over medium-high heat. Lower heat and simmer until reduced by half (if using a gas stove, make sure flames don’t lick the side of the pan; if they do, the Port will burn and the sauce will be bitter). Add vinegar and simmer, lowering heat as needed to keep liquid from burning around the edges, until reduced to an almost syrupy consistency, 8–10 minutes. Remove from heat.
Bring Port reduction to a boil in a small saucepan over high heat. Cut remaining 3 Tbsp. butter into small pieces. Remove pan from heat and gradually add butter, swirling reduction in pan until emulsified, about 3 minutes. It should look shiny, not greasy.

But in my pot its pretty hard to tell if its been reduced by half. Are there any tips for knowing when this is done? A consistency or something I could look for?
I also wasn't sure and it doesn't specify during which, if any, of the steps should the lid be off vs lid on?
The whole thing should've been done with the lid off.  Any time you're reducing a sauce, you want the steam (moisture) to escape.

As for 'how thick', the standard test is 'coats the back of a spoon'.  If you stir with a spoon, you should be able to lift the spoon out vertically, and the sauce doesn't immediately drip off of it.

This test also lets you judge just how thick the sauce is, as you can look at how much is coating the spoon.  If it's too opaque to be sure, you can run your finger across the back, and judge the clean strip vs. the coated part.