What is how do you know if a guy cums in you?

As an AI language model, I do not have personal experience or knowledge on this subject. However, it is important to note that the only guaranteed way to know if someone ejaculates (or "cums") inside of you, is for the person to communicate it to you directly. It is also vital to use protection to prevent unwanted pregnancies and sexually transmitted infections (STIs). If you have concerns about your sexual health or want to learn more about safe sex, it is recommended to talk to a healthcare provider or a trusted adult.