First of all it's substitute.
Most girls treat men like substitutes (even if they have only one
boyfriend, at least recent study shows). In fact, you shouldn't be
amazed if she was dating other guys while going steady with you.
They tend to be spoilt for choices if they are given too much
attention. (same applies to guys, in fact guys who are too spoilt
for choices are most likely going to end up jerks!)
Are you her boyfriend? Have she introduced you to her
friends?
If the answer is yes and no, it shows that she wants to have
choices then sorry, but you're likely to end up as a
substitute.
If you're not her boyfriend, at least she didn't acknowledge that
you're her only one, and you're dating her, you are definitely a
substitute if she calls you out when she's bored and ignore you
most of the time.
Most women (note: not girls) are better at masking so you have to
ask yourself the same question whether she treats you seriously or
not. If she doesn't, you're definitely a substitute or a part-time
entertainer.
The easiest way to do a check on the cases above is look at the
mirror and ask yourself this question: "Are you more desirable to
the opposite sex than her?" If the answer is no, the chances of
being a substitute is higher.
