A guy's behavior after a first date is a tell-all. If he's not interested in you, he's not going to contact you for a second go-round on the dating wheel. If he doesn't come right out and tell you that he likes you after your first date, you can try to read the signs and figure out if he might be contacting you in the near future.
If the guy you're with happens to linger at the end of the date, staring longingly into your eyes, moving closer and hanging onto every word you say, you can safely assume that he likes you. If his parting words include a suggestion of doing something together in the near future -- such as "When will I see you again?" -- you can also assume that he likes you.
A sure sign that a guy likes you is if he calls or texts you that same evening or the next day after you had your first date. That means he's thinking about you and he's interested in at least talking to you again, or he may even go ahead and ask you for a second date at this time. If he wasn't interested, he would not waste his time calling you.
He Makes the Most of Opportunities
A guy is into you after your first date if he calls and asks you to do something like run an errand with him or he asks you to meet him for lunch during the workday. If his interest wasn't piqued, he wouldn't even be thinking about including you in his daily tasks or making an effort to fit in a workday lunch with you.
If a guy you went out on one date with doesn't call you within a couple of days of the date, chances are that he's not interested in you. Yes, you can call him and feel him out, but the only reliable way to tell if he likes you is to wait and see if he calls within a couple of days. If he doesn't, there could be a good reason like a family emergency or a sudden, serious illness -- in which case, he will contact you sooner or later if he likes you. If he owns a cell phone, he can basically get in touch with you anytime and anywhere.