Yeast Starter: did it turn out?

How will I know if my yeast starter worked and if I have enough?  Here is my starter in a growler after 2 days.

Lots of questions to determine that.  What is your batch size, how many liquid yeast pouches or vials did you pitch to starter.  Ale or lager.  OG target.  What volume or diameter is your growler.  Did you stir or shake the starter.  If you take a pic from table level with a coin or something next to it for reference it might be easier to see.  Is that foam (or something else) stuck to the sides of the growler or is it floating?

I pitched a three-day swollen smack pack of Wyeast 1056 into 3 cups of 1.020 wort. Here is a close-up with reference penny. I shook the heck out of it for 5 minutes.  Yeast on the sides, but starting to worry my starter is infected from the look of the foam...

Don't worry if its infected. Just keep it covered with sanitized foil.  But your starter gravity and volume were both too low.    If you go here http://www.brewersfriend.com/yeast-pitch-rate-and-starter-calculator/you can enter the info in the screen shots attached. In part 1 you will need to change the wort gravity and wort volume to whatever you are working with from the beer recipe.  Then change the date to match the age of your yeast.  Click update.  Go to step 2.  Click 'grab from above'.  In Part 2 Step 1 enter the values from the screen shot (I converted your 3 cup 1.020 starter to .71 liter) and you will see you didn't grow enough yeast.  Go down to step 2 and change the volume to what you can easily fit in your growler.  You should decant the used up liquid portion from the growler and add fresh wort  at the volume and gravity you get from the calculator.  Leave the 1.020 spent wort in the growler till just before you add fresh wort. The calculator will tell you if its enough volume to get to your cell count needed. Make sure you click update when you change the 1.5 liter volume I entered on the screen shot. If its not just click on step 3 and repeat the process.  Not sure the volume of your growler, but next time if you use something that will hold 2 liters and increase your starter gravity you wont need additional steps, at least for standard ales.

Thanks for the resource! 

I think I picked a bad recipe to try out liquid yeast (OG 90). What will happen if I'm just shy of the mark?  Can I add another smack pack if my fermentation gets stuck?

If you plugged your data into the calculator, and used its results, fear nothing.  There is no such thing as a bad recipe for liquid yeast. Adding another smack pack will get you nowhere.  Post here first, someone will help.

Looks fine. Don't worry. Just pitch it.