Ask the right questions before hiring. Learn enough about the subject to ask good questions. Ask how they would solve an adjacent problem.
Set checkpoints for the contractor's work. Have the dev build something partial that you can test. Build another and test it. Integrate them and test them. Wash, rinse, repeat.
Learn the language, or else. You could get frustrated and waste time, or you could get fleeced. You can't effectively communicate with an engineer figuratively speaking another language.
And communication is everything, especially for remote work.