What units are used to measure beer by volume? It may be dependent upon local custom, but what scales are used to measure beer? Is there one in particular that is used globally, or could be said to be specific to beer or alcoholic drinks?

Typically, I see either pints or whatever the local standard of measurement for liquid is (say, fluid ounces or milliliters) so this isn't really a global standard.

Serving sizes also aren't standard; in my country alone, if you go to a pub and ask for a pint or a schooner, the size will depend on what state you're in.

However, I have seen the measurement l (lower-case L) used. Is this litres, or something else?
Beer is measured by volume, but which measure depends both on country and on the context in which the measure is being used.

For instance, in the US, you will almost always find servings of beer measured in fluid ounces. 

However, in production, you will regularly find the measure being listed in barrels (31 imperial gallons).

You'll regularly find beer sold in pints (16oz), though that could also mean a 20oz glass as you only get 16oz of beer with a proper head.

My limited experience with bits of Europe indicates beer there is sold by the mL. Also worth noting is that Liter can be represented as either L or l so when you see "l" you're seeing Liters.

So, no. There isn't a global unit of volume for beer. But it is almost always sold by volume.

Also, with the rising popularity of high gravity beers, standard beer servings vary in part because to serve similar amounts of alcohol across standard servings you've got to serve differing quantities.