She refers to you as a "friend" too much and makes it clear in sneaky ways. Does things to only benefit her. You make more of an effort to share your feelings or plan things out. You call more. You find yourself waiting for her next move. I went through this for three months and in the end....I got hurt pretty bad.
This sounds like a sexuality question! (hee hee, couldn't resist)
But assuming you mean emotionally, if you're in a relationship with someone, you're filling a void. So are they. It is the void you felt when you didn't have a relationship, but wanted one. So start there.
I assume you mean, in addition to the loneliness, desire for companionship, love, connection, fun dates, all that, are you being used to "fill additional voids" that you'd rather not be filling, right?
Like, rebounding from a prior broken relationship needing you to just feel attractive again? Or someone whose too insecure to be independent and would prefer you around to being single no matter how poorly suited you are for her? Those seem like good voids to avoid, if you know what I mean.
For rebounds, what is her history prior to you? Long time single hood or did she just get divorced? Or even worse, are YOU the reason she is going through a divorce as we speak? If so, you're filling the rebound void, most likely. Or if she talks all the time about her ex, she's still with him in part, and just using you. People over their exes hardly ever want to bring them up with their new guys unless there is some void filling going on.
As for the insecurity void, well, how insecure is she when without you? Can she go out without you? Have a good time? Feel confident? Spend time alone without you checking in and pandering to her? If so, she's probably reasonably healthy and void free. If not, she's got emotional issues that, trust me, YOU'LL never fill. Instead, she has to learn how to be happy with or without you, THEN choose you. Otherwise, she'll stay unhappy with you and BLAME you. So stay out of that void.
"How can you tell if a girl is just using you to fill a void?" -- If you have to ask the question.
Furthermore,... even if she isn't, if she makes you FEEL that way, is this really a relationship that you want to be in?
Hard to tell. Perhaps if you elaborate on why you felt like you were used it would strike a cord with others and they could add their opinions.
To me, I'd feel like I was being used to fill a void if it a one sided relationship. For example,
* only talks about herself, her past, her future - devoid of you
* she never inquiries about you or quickly changes the subject from you to her
* only around when she needs you
* talks about one day finding true happiness and love - devoid of you
* Has dependency issues, is on the rebound and keeps withdrawing from your emotion bank without making any deposits - takes, takes, takes.