How to choose the right Citi Bike?
Typically you would want to do an ABC Quick Check - the information below came originally from the League of American Bicyclists site.

A = air


Inflate tires to rated pressure as listed on the sidewall of the tire.
Use a pressure gauge to insure proper pressure.
Check for damage to tire tread and sidewall; replace if damaged.


B = brakes


Inspect pads for wear; replace is there is less than ¼" of pad left.
Check pad adjustment; make sure they do not rub tire or dive into spokes.
Check brake level travel; at least 1" between bar and lever when applied.


C = cranks, chain and cassette


Make sure that your crank bolts are tight; lube the threads only, nothing else.
Check your chain for wear; 12 links should measure no more than 12 1/8 inches.
If your chain skips on your cassette, you might need a new one or just an adjustment.
Note - most of these bikes use Nexus type internal hubs, so just a visual check of the chain and a quick test ride should let you know what the condition is.


Quick = quick releases


Hubs need to be tight in the frame; your quick release should engage at 90°.
Your hub quick release should point back to insure that nothing catches on it.
Inspect brake quick releases to insure that they have been re-engaged.


Check = check it over


Take a quick ride to check if derailleurs and brakes are working properly.
Inspect the bike for loose or broken parts; tighten, replace or fix them.
Pay extra attention to your bike during the first few miles of the ride.


Edit: The comments seem to indicate that some people think this list if overkill. However, take a step back and remember... You are picking out a bike that doesn't belong to you, that you don't maintain and that really you know nothing about. True, you probably aren't going to measure the chain, or have a pressure gauge in your pocket, but I would spend MORE time inspecting a citibike before I ride off than I would a bike of my own. I am certainly going to look fairly close to see if anything is loose, broken, or way out of adjustment, and the mnemonic ABC Quick Check is an easy way to remember what to look for.