The phrase "good guesser" means that someone is good at making a correct or close estimate or prediction, even if they do not have all the facts or information. Full definition
Unless you're a really good guesser — like you always know the number of jelly beans in the jar at the state fair — you'll either underestimate your need, leaving your loved ones unable to cover expenses, or you'll overestimate and pay a lot more for your policy than you need to. (policygenius.com)
Then various ideas to see who is best guesser - most right, closest,... eventually finding standard deviation of their guesses. (tes.com)
Given that most of the exam is multiple choice, a good guesser who can do the simpler questions will pass. (hechingerreport.org)