This potential is often evaluated using formal aptitude tests, which predict the degree of success the candidate will have with a new language. Aptitude tests vary but many include evaluation of ability to manage sounds, grammatical structures, infer rules, and memory.
Example
The Modern Language Aptitude Test (MLAT) evaluates language aptitude.
In the classroom
Language aptitude may be fixed but there are many things teachers can do in the area of learner training to improve the learner's ability. These include helping learners identify their preferences for learning; thinking about learning styles, and then looking at how these can be developed; and developing learner autonomy by teaching learners how to study effectively.
Further links:
https://www.teachingenglish.org.uk/article/lizzie-pinard-fostering-learner-autonomy
https://www.teachingenglish.org.uk/article/learning-styles-discussion-forum