Answer
Through the 'early warning system' — they can issue reasoned opinions; enough objections trigger a 'yellow' or 'orange card' requiring the proposal to be reviewed.
💡Based on Protocol No. 2 on subsidiarity and proportionality.
Consolidated TFEU (EUR-Lex) — eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:12016E/TXT