Detect a Hidden Foe
Your Wisdom (Perception) check lets you spot, hear, smell, or otherwise detect the presence of a hidden enemy. It measures the keenness of your senses. (82) Go here for examples.
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Poor lighting, fog, and other obstructions may give you disadvantage.
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Moving at a fast pace can also give you disadvantage.
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Natural obstructions may limit how far you can see.
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Listen Check: There are limits on how far you can hear.
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See DMG 244 for Tracking DCs.
An invisible creature can always try to hide. Signs of its passage might still be noticed, and it does have to stay quiet. (80)
Passive Perception. Even if you are not actively searching, the GM can determine whether you detect the presence of a hidden enemy in passing through use of a passive Wisdom (Perception) check. Go here for an example.
Using magic. Sometimes you can use spells or magic items to help you discover a hidden foe.