You try all 2^32 possible nonces. On realistic mining hardware, that takes a very tiny fraction of a second. Of course, most of the time there is no nonce that works. Mining is so hard that all the miners combined only find a block that has any nonce that meets the difficulty once every ten minutes.
For a given block header where the only variable is the nonce, it is trivial to brute force all nonce values and verify if any gives a hash that meets the target.
For the vast majority of block hashes, no nonce value will meet the target in normal difficulty settings (let alone at a target of 3).
Miners get around this by altering other values in the block ...