They are different in usage. Usually, we would use the first phrase. Some examples of how to use "guess" are as follows: They guessed that it would be right. They took a guess at this. Take a guess. Guess. Can you guess the answer? Hope this helps :)

Examples... Take a guess at the answer. I guess that it will be sunny later today. "At" can be used depending on the context. We will not say " I guess at it will be sunny..."