TMI but how do you know you're about to orgasm?

So my SO other and I have been together for a while. We haven't had sex, but that's okay for both of us. And I can always get him to orgasm, but when he tries to get me to, I just can't. This may sound weird but I get this feeling that I sorta have to pee, but it isn't. I'm just afraid that I actually do have to pee and its not an orgasm. Is that a sign that I will? I'm sorry if this is weird, but I just don't want my SO to feel like he's not doing a great job. He totally is though! I just can't orgasm. I've done it before in previous relationships, so why can't I now? Do I just worry too much?