Start with letter counts
To unscramble letters without guessing, count the letters you have, look for fixed patterns, try likely word lengths, and reject any word that uses letters you do not have.
A candidate word is only valid when every letter can be supplied by your input. This is why duplicate letters matter: one t cannot make a word with two t's.
Use patterns after the first pass
If a board gives you a starting letter, ending letter, or contained letter, apply that filter after the broad search.
Try Word Helper
Use the Word Unscramble tool for broad words-from-letters results, then scan by length.