Biden won by a 4.4% margin.
As to the ridiculous question of how can you know there is no election fraud, this is a fundamentally ridiculous question as it is asking you to prove something that is impossible to prove.
All that we know is there has been no valid evidence of fraud presented. OrechDin summed it up very well. If you still persist in lies, then like Lyndsay Graham, a fanatic Trump supporter, said about this there is simply no convincing you.