Most sex assault accusations are near-impossible to prove as true or false because of the inherent nature of them. No other witnesses, etc. 

Some accusations can be proven false, however. For example, by way of the accuser being caught conspiring or bragging about the false accusation in private messages (this happens in both criminal and family law; text conversations sink people's stories all the time). Or the accused has proof he wasn't with the accuser at all during the time of the alleged offence. It does happen. 

Look at that one Ghomeshi accuser. She testified under oath she never saw him again after the assault, never wanted to see him again, and deliberately avoided him. Then she was busted under cross with her own text messages to Ghomeshi the day after the alleged assault, singing his praises, complimenting him and pleading to meet up again for another date. That didn't prove the accusation false, but it sunk the Crown's case because it proved her a total liar under oath.