There are several clues that you may be near an animal. One is noise such as rustling in the fallen leaves or in the trees. Some animals seem to make no noise that I ever hear. Others issue forth warning calls, or let you know if they're upset. Another clue is odor.
Some animals give off distinctive scents or odors that are easily noticed by humans.
Strong scent may mean that a particular animal is close by or has passed through the area recently. I'm learning that in the dark, the best indicator that an animal is close, is seeing their eye-shines. As we walk, we slowly move our heads from side to side and up and down, scanning the forest as far as our headlamps project light. When we see one or two small yellow circles of light glowing through the trees, sometimes seeming to stare back at us, we know we may have spotted an animal. We investigate further by using our binoculars and the spot light.