Moon Page makes it easy to look up the phase of the moon for any date. By default, the current moon phase (EST) is displayed; to find any past or future lunar phase, specify the date and time, then click "Find Moon Phase." (Before submitting, you may choose Universal Time or a North American time zone.) For next or previous new/full moon, click the appropriate button.
At 13:56:51 UT on Thursday, 23 May 2013
the waxing moon's phase is 96.23% full.
(Or, to be more precise, 96.231635007486% full.)