Well, we or someone else could talk you (or give you reading info) through some of the things people often feel when that's happening -- things like muscle spasms, for instance -- if that's helpful. But other than that, it's just mostly about paying attention to your body and your sense of yourself with things that do often happen with orgasm in mind and making your own call on it.
Sometimes orgasms are super-mild, mild enough that we probably don't even notice some of them, but other times, it's pretty undeniable. However, even that range of experience varies with people a lot. Some people, for instance, may find that any kind of orgasm feels really intense to them, or that it almost always does -- others can be the opposite.
The good news is, we really don't have to know if we or someone else reached orgasm during or with sex anyway, not really. We just mostly need to know if everyone is having a good time, enjoying themselves, feeling good and getting what we all need to feel satisfied, you know? So, no worries if you can't say for sure for a while yet still: it's not required to have a good time or learn what you enjoy and want most.