Hydrometer

Put your liquid in the hydrometer tube, spinning the hydrometer and measuring the original gravity (OG) when it settles. Try to make sure there is as little carbonation (bubbles) as possible in this process.

Once you think it's done fermenting (when the lavalamp-esque activity stops, usually within 4-12 days), take another measurement with the hydrometer for final gravity (FG).

(OG - FG) * 105 = {% Alcohol by Weight}
{% Alcohol by Weight} * 1.25 = [% Alcohol by Volume]

If we take the first case from the Cider example, the OG was 1.050 and the FG was 1.006.

(1.050 - 1.006) * 105 = 4.62
4.62 * 1.25 = 5.775

Thus, the Cider with the EC1118 yeast has approximately 5.8% alcohol.

Interesting note: Although the rumour is that American beer is weaker than other countries, they often use ABW where many other countries will use ABV. As you can see, there is a significant difference between the two.