We don't know until after the election, and maybe not even then given Project Veritas publishing revealing video of Democratic leaders admitting to the vote fraud that everyone has suspected. The fact that Democrats have fired the people who made the admissions confirms that the statements were true. And they are no longer saying vote fraud is rare, but instead saying "if there is a lot of it, blame the Russians".
This means Trump has to win not just with a majority but a large enough margin to offset Democrat vote fraud. Given Democrats already sending in mail in ballots for dead people, admissions of having minorities vote multiple times, sending instructions on how to send in your mail in ballot and then vote in person - Trump could need to win by 1-2% more to get the numerical victory over Clinton.
End result - unless Clinton wins by 5%+, her victory is likely a fraud. And Democrats have already said that if Trump wins, they'll blame the Russians and consider it illegitimate. So victory at the polls unless by an incredible margin (and maybe not even then), neither side will or should accept it.
Trump’s Right – The System Is Rigged And We Don’t Owe It Our Default Acceptance