Learn Spanish (Latin America): ¿Pueden ver aquellos valles?

Both mean "those", but Spanish has two ways to say "that" or "those". Esos is used for things somewhat close to the listener, while aquellos is for things far away from both people (like valleys you are pointing at miles away in the distance).

Since valles ends in -es, how do we know it's masculine and needs aquellos instead of aquellas?

Nouns ending in -e (the singular is el valle) don't follow a strict gender rule like words ending in -o or -a, so you just have to memorize their gender! Since valle is masculine, its plural needs the masculine plural adjective aquellos.

Who is doing the seeing in this sentence since there is no pronoun?

The verb pueden is the plural form of poder (can / to be able to). It can mean "they can" or "you all can" (ustedes). Without a pronoun included, everyday context implies you are addressing a group of people directly: "Can you (all) see those valleys?"

Why is ver not conjugated here?

When you have two verbs working together to express one action (like "can see"), only the first verb (pueden) is conjugated to match the subject. The second verb (ver) stays in its infinitive (dictionary) form.