You rarely miss her
At the start of your relationship, you two were Velcroed-at-the-hip. You rearranged your schedules so you could spend every free moment together, were happy to do everyday things like grocery shopping together because it meant more time shared, and neglected your friends to the point that they cracked jokes about you two being conjoined twins.
Of course, no couple can maintain that high level of attachment forever (and if they did, it would be called co-dependence). But, if you find yourself seeing her less and less, and are completely fine with it, or worse yet, enjoying your independence more than your time together, it's a serious signal that your interest in the relationship is seriously waning.
Spending time apart is healthy, but if you find yourself in an extended John Waite phase, and you ain't missin' her at all, it might be time to let her go so she can find someone who will.