How can I find out if my girl is talking to someone else?

A single slow reply means almost nothing. People get busy, have bad days, forget to answer. What matters is the shape over two or three weeks. Are plans still being made, and by whom? Does she still ask about your day? Does she still bring up things from earlier conversations? If the effort is roughly even and she is still choosing to spend time with you, the other-person theory is probably not the main story.

The signs that point more clearly toward something happening elsewhere are sharper: she is consistently unavailable on the same nights, her replies come in bursts that look like she is fitting you in, she stops offering specifics about her week, or she gets defensive about questions that used to be easy. None of these prove anything. Together, over time, they tell you she is giving you less of herself, which is the thing you actually have to respond to.

When you want to check where you stand, the most useful move is usually to name what you have noticed in terms of your own plans and feelings, then ask one plain question. The message should be short and it should leave her an easy way to answer. Something that describes what you expected and what happened, followed by a direct but open question about where she is at. The draft should carry no lecture, no list of grievances, and no hint that you have been watching her online.

Leave out anything that depends on information you got from outside the conversation. If you saw her with someone or heard it from a mutual, do not reference it, because it turns the whole exchange into an argument about surveillance. Keep the focus on what she has told you and what she has done in front of you.

Send the honest message in daylight, when you are not annoyed or tired. Then leave it. Waiting a day or two is normal, and a considered reply is often better than a fast one. If she answers warmly and clears things up, you have your answer and can move on. If she gets vague, deflects, or disappears for a week, that is also information.

If you get no answer, send one short follow-up at most, something that keeps the door open without chasing. After that, stop initiating for a while and see whether she comes back on her own. If she does not, you can stop guessing. Someone who is still fully into you tends to close this gap without being pushed, and someone who is not will show you that by how little effort they make to explain.