A call holder who exercises the day before the ex-date owns shares on the record date and receives the dividend. The assigned seller is short the shares on that date and owes it. This is the single most common cause of unpleasant surprises for covered-call and spread sellers.
The check is mechanical: compare the dividend with the short call's remaining extrinsic-value. If the dividend is larger, assume assignment and roll or close first.
Example: XYZ at $52, ex-dividend $0.75 tomorrow, and the $45 call trades with $0.10 of extrinsic value. Exercising captures $0.75 and costs $0.10. You should expect assignment on essentially the entire open-interest, and you will owe $75 per contract on top of the share delivery.
Related: early-assignment, early-exercise, dividend, covered-call