How it's checked:
• Examiners look at range and accuracy of grammar:
◦ Simple vs. complex sentence structures
◦ Verb tenses, articles, prepositions, agreement
◦ Error frequency and whether errors affect understanding
• Grammar is one of the 4 scoring criteria in Writing ("Grammatical Range and Accuracy").
This is why practicing grammar is still important for the IELTS, despite there being no direct multiple-choice grammar questions.