You're welcome.
This
site will help you figure out when a
preposition is needed after a verb (and when it's not).
You'll find
mirar listed in 'Verbs followed by an object' group, under the 'Verbs with no preposition' link.
Remember, the personal 'a' is not a preposition. So, if the object is a person, the verb will be followed by the personal 'a'.