Well, this is all from my own personal experience. I will try to tell you how to tell from both my perspective (male) and my cousin's perspective (female), although keep in mind that the female perspective is just what I witnessed, and may have not been intentional.
My feelings for my half-cousin first began, I think, when she was eight and I was thirteen. I just remember that I stayed the night and slept in her room on the floor, but before I could go to sleep I just watched her as she slept, thinking how pretty she was. Then, when she was twelve and I was seventeen, her body began to develop, and I became physically attracted to her, at least more than I already was. I tickle all my little cousins and nieces and nephews, but when I tickled her I did so because I enjoyed being close to her. Like, if there was a choice between tickling her and tickling a different cousin, I'd choose her. I don't know if that helps. So as she grew, turning thirteen and fourteen and fifteen, I noticed she started wearing less and less clothing around me. Like, she'd wear really short shorts and low-hanging tops with no underwear. She'd always sit next to me, lean against me. She'd have me hold her during scary movies. She'd be laying on the couch, and she'd lift her legs to let me sit, then put her legs back on me and ask me to tickle them. Which I would, of course. She'd play that dancing video game for the Wii, I forget what it's called, and just jump around in her short clothing. On my part, I would watch her, look at her body, her lips, and I wasn't sure if she noticed. Although I was pretty horrible at hiding my glances, especially since my eyes would linger for several minutes while she was looking elsewhere.
Then, when she turned sixteen, she got her first serious boyfriend. Needless to say, I was jealous. He'd come around the house during holidays, and I'd be nice to him and whatnot, but I didn't like him. And then he broke up with her and she cried for days in her room, and I wanted to KILL HIM, RIP HIM APART! Even though I was glad she wasn't seeing him anymore. My original plan was to wait until she was eighteen before I told her how I felt, so that way she'd be an adult, she'd have gone through her high school phase of liking every cute guy. But sometime after she turned seventeen, she got another serious boyfriend. And I was jealous again. He'd spend the night, sleeping in her room with the door closed. I was amazed her parents let her do that. She'd lay on the couch with him, wrapped in his arms, and I'd just be fuming silently in the corner, pretending nothing was wrong.
Then, one day we shared our first kiss, as I mentioned in that other topic where you describe how it felt the first time you kissed your cousin (although, at this time, I cannot find my message, but that may just be a first-time-user screening process thing; I'll know when I post this message). And she told me it was about time, then texted her boyfriend to break up with him. As it turns out, she had been doing that whole 'public displays of affection' think with her boyfriends in front of me to make me jealous so I'd kiss her. She knew I would look at her body, which she liked, which was why she'd wear very little around me. She was not so scared during the several horror movies we'd watched together that she just grabbed the closest person for comfort, she consciously grabbed me for comfort. She loved it when I tickled her for the same reason I loved to tickle her.
So, in summation, you just have to read him/her. If she's free with her body around you, that may be a sign. If she enjoys being touched by you, that may be a sign. If she likes to make you jealous, that may be a sign. If he checks you out, that may be a sign. If he finds reasons to touch you that may be innocent, that still may be a sign. If he comforts you when you're scared, that may be a sign. If he wants to beat up or otherwise physically harm guys who have broken up with you, that may be a sign. Note: may be a sign. Because, as has been stated before by many people, there is no sure-fire way of knowing whether or not someone likes you unless A) they outright tell you or you outright ask them.
Peace from the Ents, TreeBeard.