I often hear from questioning people the most useful thing to see is a list of common ace experiences – so here is one such list. (Please use this with care. These experiences aren't universal and non-asexuals will relate to some of them.)
(1/13)

• finding people aesthetically appealing, but that's as far as that feeling goes
• the idea of sex never occurring to you on its own
• finding conversations of a sexual nature especially boring
• consistently not initiating or suggesting sex with your partners
(2/13)

• deciding to 'put up' with sex because it seems like a requirement to have an intimate relationship
• your ideal relationship would be one that doesn't include sex
• having sex but 'not getting what all the fuss is about'
• being repulsed by the idea of sex
(3/13)

• pursuing sex as an intellectual curiosity rather than due to attraction
• feeling like you could go the rest of your life without sex just fine
• not feeling that sex is much different to masturbation
(4/13)

• pretending to find people attractive when a friend asks
• saying who you think is attractive by guessing what other people would think
• wondering why everyone else seems to find sex so interesting
(5/13)

• being confused when other people's fantasies include sex
• forgetting or not realising that other people think about sex
• being unable to relate to the idea that someone could 'need' sex
• not understanding why people find abstinence difficult
(6/13)

• not understanding what would ever motivate someone to cheat in a relationship;
• wondering why people pursue sex when it seems to just be messy and something that complicates relationships;
• feeling like people place too much emphasis on sex in relationships
(7/13)

• not understanding what it is about sex that makes cheating in a relationship particularly frowned upon compared to other activities with another person outside the relationship
• finding flirting confusing or failing to even notice it
(8/13)

• not understanding why people seem to think romance
• not seeing why people act as if cuddling and/or sleeping in the same bed implies a sexual relationship
• thinking kissing is strange and not understanding why people would want to do it
(9/13)

• wondering how people would have first come up with the idea of sex before modern society existed to tell them about it
• appearances of sex in fiction often seeming random, out of place, or uninteresting
(10/13)

• thinking that everyone is exaggerating or ironic or being 'immature' about sex and that really they all see it the same way you do
• not understanding / thinking it's a joke when people say they would have sex with a certain stranger
(11/13)

• not realising that sex dreams are real or happen as often as they do
• thinking that people only involve others in sex because of social expectations
(12/13)

• thinking "I'd know if I were gay so I must be straight";
• thinking "I'm not attracted to the opposite gender, so I must be gay"
• thinking "I feel the same way about both men and women so I must be bi/pan"
(13/13)