Here's how someone got to this blog. They searched for...
how to tell if someone is having consensual incest
Well, one way would be to catch them in the act. However, if that involves snooping, barging in to private areas, being a peeper, etc... well, that's not so good.
Another way would be to notice if they're always smiling and glowing like they've having the best sex ever. Just kidding. Kinda.
Seriously, observing how they behave around the person(s) you think they are secretly with or how they talk about that person might give it away. When people are in love, it can be hard to hide.
The question I have is, why does this person want to know? If it is just curiosity, assume whatever you'd like to. Just don't talk about it to others because it really isn't anyone's business, and since consanguinamory is still criminalized in many places and consanguineous lovers are otherwise subjected to discrimination, they shouldn't be outed (unless they are someone, such as a prosecutor or legislator, perpetuating discrimination against other consanguineous lovers.)
If someone wants to know because they want to be supportive, that's another matter. Here's the advice I have written for people who are friends and family of consanguineous lovers. The Final Manifesto has this excellent advice for friend and family of consanguinamorous siblings.
The trick to getting someone to confide in you that they are involved on consanguinamory is to signal to them that they can. The way to do this to convey the message that you 1) do not condemn consanguinamory, 2) care about them, and 3) can keep a secret.
Communicating #1 can be as easy as saying something like (especially if you have one of them alone), "I was reading about a country that is considering repealing laws against consensual sex, particularly a law they have against close relatives having sex. I was surprised some places still have laws like that. I mean, if they are consenting adults, what's the big deal?" Or if you live in the US, you can say something like "I was reading about how different states have different laws. I was surprised to read that a couple states still criminalize consensual sex between first cousins, and that only a few states do not have any laws against consensual incest, no matter how closely related. I would think most, if not all states, would have repealed such ridiculous laws against consensual sex."
The worst-case scenario is that they are not only NOT involved, but they disagree with your statement, in which case you can ask them why they disagree. If they accuse YOU of wanting to be involved, you can point out that someone doesn't have to secretly want someone of another race to support interracial marriage, or someone doesn't have to be a woman to support women's rights. Otherwise, they may shrug their shoulders or agree, but not reveal anything to you (at least not right away.) That could be because they aren't involved. Or, they may open up to you, which would be great because most people in consanguinamorous relationships really appreciate support and someone to talk with.
There are other ways of communicating #1 or furthering the point, such as saying you read someone talking about these books, or saying you were reading about how common it is and that everyone knows someone involved, and asking, "What do you think about that?"
Another way is saying that you notice that in discussions about marriage equality, some people want to keep denying close relatives their freedom to marry and you just can't think of a good reason why someone would want to stop people from being together if they love each other.
Another way that might work, if their facade is that they aren't in a relationship with anyone, or they've been mysterious about who they are seeing, is to say something like, "I want to be happy. If you're happy not being in a relationship, I support you. And if you're in a relationship, as long as everyone involved in a consenting adults, I support you."
The trick is to give them enough wiggle room to pick up the ball and run with it, if they are ready to. They may not, or may not do it right away, but there are ways of letting them know you would be supportive that are not likely to give someone who ISN'T involved the idea that you think ARE involved. You can also prevent a misunderstanding by saying something like, "I don't have any secret crushes on relatives myself, but it isn't wrong to have them."