grammar - When is "to" a preposition and when the infinitive marker?

• I want to see you.


• I look forward to seeing you.

How can one say "to" in the first sentence is an infinitive marker and in the second sentence a preposition when we are given just the following two sentences and are asked to fill in the blanks?

• I want to ____(see/seeing) you.


• I look forward to __(see/seeing) you.