She says she’s not looking for anything serious.
This isn’t always soobvious. Sometimes she’s lying because she thinks you’re not looking for any commitment, and she doesn’t want to come on too strong. But often she legitimately means it. If she says it without you initiating any talk of your commitment-phobia, it’s less likely she’s just trying to convince you it’s all good—and more likely she’s just looking for casual, convenient s*x… or that she just likes hanging out with you.
She won’t introduce you to her friends or family.
If it’s been a while and you’ve yet to meet the people closest to her, it’s not because she’s nervous, which she might tell you; it’s because she doesn’t care enough that she needs affirmation from them. If you were a serious prospect, she’d want their opinions as soon as it wasn’t creepy to invite you to meet them.
She avoids your pursuits—but not all the time.
If you find that you’re the one always initiating plans, or that she’s flaking on your plans, it’s probably because you’re a second, third or fourth option—which is why she’ll sometimes kick it with you. It’s a harsh reality, yes, but if she really wanted to be with you, she’d want to hang out with you, and she’d probably ditch her other plans to do so. Of course things do come up, but if canceling becomes a regular occurrence, make no mistake: She’s not looking to seriously date.
She tells you about other men.
Unless she’s clearly trying to make you jealous, she wouldn’t be telling you about her rendezvous with other dudes. If she does, it’s probably because you’ve been friend-zoned—even if you’re sleeping together.
She doesn’t initiate the commitment talk.
In the same vein, if she really wants to be with you, it’ll keep her up at night wondering if she’s the only one you’re seeing. Eventually, she’ll ask you about it. If months go by and she doesn’t, it’s because she doesn’t care whether you’re dating or sleeping with other people, because she is, too.