WordNet sense 1 (a message expressing an opinion based on incomplete evidence):

conjecture, hypothesis, speculation, supposition, surmise, surmisal

WordNet sense 2 (an estimate based on little or no information):

guesswork, shot, guessing, dead reckoning

WordNet sense 3 (put forward, of a guess, in spite of possible refutation):

hazard, pretend, venture

WordNet sense 4 (judge tentatively or form an estimate of (quantities or time)):

estimate, gauge, judge, approximate, approximate, estimate

WordNet sense 5 (guess correctly; solve by guessing):

infer

WordNet sense 6 (expect, believe, or suppose):

imagine, opine, reckon, suppose, think