Learn Italian: Sono sempre più meticolosi al confine. - They are increasingly meticulous at the border.

While sempre means "always" and più means "more," when put together as sempre più, they form a fixed expression that describes an ongoing, progressive increase. In English, this is most naturally translated as "increasingly" or "more and more." For a progressive decrease, you would use sempre meno (less and less).

Since sono can mean both "I am" and "they are," how can we tell which one it is here?

The clue is in the adjective meticolosi. Because it ends in -i, we know it is a masculine plural adjective. If the subject were "I am," the adjective would have to be singular: Sono sempre più meticoloso (or meticolosa). The plural adjective confirms the subject is the implied "they" (loro).

Could I use alla frontiera instead of al confine?

Yes, in everyday speech they are practically interchangeable when talking about crossing borders. Strictly speaking, il confine refers to the exact geographical or political boundary line, whereas la frontiera refers to the surrounding zone where the customs and passport checks actually happen.

How could I apply this lesson's topic (emphatic exclamations with a pleonastic "che") to this sentence?