How healthy am I?

Solution 1:

You can see your exact HP, in the statistic screen, under details. This can be accessed by pressing TAB, and pressing the tab the furthest to the right.

However, in the heat of battle it's a lot easier using the visual indicator, instead of going into the menu.

As you can see from the image below, the indicator shows that I'm very close to death. After a bit of testing, I've got around 10-15% HP left. So the wider circle of red, the less HP you have.

Solution 2:

I've yet to see any true Health bar that indicates your actual health. I don't know if there is maybe a way to turn something on in the PC version or not.

The only indicator that you get for sure is the visual cues of the screen turning red from the outsides of the screen towards the middle, and the heavy breathing from your character. Your best bet in a case where you're hurt is to find some cover and let the nanotrites heal you some, or, as you said, use a bandaid. If you happen to die, and are lucky, you can use your defibrillator if it's fully charged.