Detect a Secret Door or other Thing
When you actively seek evidence of a hidden or secret door, portal, compartment, or other thing, you make a Wisdom (Perception) check against the DC specified in the hidden thing's description. This check measures the keenness of your senses. (82) Go here for examples.
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Moving at a fast pace can give you disadvantage.
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Proficiency with certain tools sometimes helps.
Certain situations may require you to make deductions based on the clues you perceive to figure out the location of a secret door, etc. The GM will ask for an Intelligence (Investigation) check instead of, or in addition to, a Perception check. (81) Go here for examples.
A lore-based Intelligence check would be appropriate if your recollection of historical, religious, or nature-based lore could help you discern the presence of a secret door, etc. (81) Go here for examples.
House Rule: You can attempt an Intelligence (Arcana) check to detect a door that is magically hidden. (195)
Using magic. Sometimes you can use spells or magic items to discover hidden things.
Passive perception. You may also be able to detect a hidden door, etc., even when you are not actively searching. See passive Wisdom (Perception) check. Go here for an example.