The English language verbs On Guard and Look Out have related meanings, but they are different, as I shall explain, using British English examples.
To Be On Guard means someone who is looking for potential danger or a threat.
"Entering the dark alley after midnight, I was on guard for thieves."
"Keep an eye on the water, we need to be on guard for crocodiles."
A Guard (noun) is a person whose job it is to protect something.
"The warehouse guard was sleeping on the job."
To Look Out means to be attentive to seeing something. This is more general than to be on guard. You can look out for something that is not a threat.
"Please look out for Daisy, I need her to come with me."
Look Out can also be used as an imperative (a command).
You saw a car coming very quickly. "Look out!" you shouted.
Lookout (noun) is a place from which you can look out.
"I posted a guard at the lookout, so we would spot the enemy well before they reached us."
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