Well you can never “be sure”, but you can improve your chances by improving the quantity and/or quality of your attack.
Quantity: Even if you think it’s a one in a million chance that someone would click your link, you can send it to 10 million people and 10 of them will still click on it.
If your going after a select target (spear phishing), really get to know the target, what kind of emails they get, what they would click on, what is important to them, who is important to them, how they communicate and how other people communicate with them.
Also, most people aren’t actively looking for phishing threats. You’ll be surprised what you can get away with. There’s a video I saw here (somewhere in Reddit) of someone asking people on the streets what their passwords are, like in a street interview, and people would eventually share their passwords. If I can find it I’ll like it here.