How do I know when to is a preposition and is a part of infinitive?

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Very good question.

A preposition is normally attached to a NOUN.

The "infinitive to" is normally attached to some type of verb or may act as the subject of a sentence.

Example:

Prepositions—always comes after a NOUN.

I gave the bike to Mike.

Kevin was send to prison.

Kevin send the letter to Mike.

Infinitive to —comes after a verb or acts like a noun and can be used in the place of a noun. (수영하는것)

I want to eat a burger.

I like to eat burgers.

I prefer to eat a burger.

I pretended to eat a burger.

To eat a burger in the morning is not healthy.

To drink soju every night is not healthy.