Basic motor skills that immediately come to mind are:

• Riding in a straight line/not weaving around
• Holding a line while looking back
• Riding for periods with one hand on the handlebars - important for signaling, etc.
• Cornering - important for making turns in traffic. (Just yesterday I saw some guy cutting wide for a turn with traffic coming up behind him and was like "nooooo....")
• Ability to safely navigate obstacles in traffic
• Proper and safe use of brakes, and for bikes with multiple gears, shifting.

If you really want to test these skills, you can set up some "courses" in a parking lot using 2L bottles, soda cans, traffic cones, etc. Assuming she's not racing crits or anything, the courses don't have to be that difficult - enough to check skills before going on the road. It might be fun to ride them with her - have her follow you, then later, lead. If frustration is an issue, start really easy and then make them harder to build confidence.

I'm also a fan of having people new to riding on roads go with a "guide" a few times. You can take her out and role model good cycling behavior, including lane position, signals, rules of the road, and so on.