You can use that phrase, but it is usually used for casual times:
It's just my guess, but I think that flower is a rose.
Because you are talking about a serious or criminal act, there should be no "guesses". Try to back it up with some facts or observations.
I suspect XX stole my money. Here's why I came to that conclusion: He was the only one in the room alone with my wallet. He showed Fred that he had a lot of money. etc.
Student or Learner