If we do come across aliens on another planet, how will we know they are intelligent or not?

We always fanatsize about finding creatures on other planets. Lets say we do find a creature (small or big). How can we tell if its intelligent (like humans) or dumb like all the other animals on Earth? Wouldn't it be risky trying to tell which they are? We might come across a 'dumb looking' creature only to find they are intelligent as us. Imagine if intelligent beings had come to Earth just 30 million years ago. How would they tell whether our alligators are intelligent or not. They would be eaten by the time they figured it out. What do you think?