There's a Hint button, just to the right of the Submit button. It is completely free, and will suggest a word you can make right now, or it will advise using the Draw Pile if there are no makeable words. The Hint button won't tell you the best word on the board, but it'll help by giving you a quick word if you if you're totally stuck.

There's also a booster called Show Best Word, which will show you the best word on the board, but will cost coins to use. You can read more about it in the Boosters section.