That's the sad thing in this case: Nothing can be proved, now or back in 1992 when the accusation was first made. Dylan Farrow (or her mother, Mia) actually made this claim many years ago, and prosecutors decided there just was not enough evidence to pursue it. The allegation only was made after Allen's relationship with Soon-Yi was made public, so there was some speculation that Mia conconcted it because Woody couldn't be charged with any crime (other than really bad manners) for the Soon-Yi stuff.
I have no idea if it's true or not, and even if it's not it is entirely possible that Dylan was convinced by her mother that it happened. Kids often have to side with one parent or the other during a divorce, and accept that parent's version of events.
You tell a story enough times, and the memories of the telling are easy to confuse with memories of actual events. And you don't have to be 7 years old for this "memory implantation" to occur: Adult eye-witnesses testifying under oath often are absolutely convinced about things they "observed" that can be proved couldn't have happened. That's one reason why detectives try to get witness statements ASAP after some crime, before they have embellished their story in telling it many times.
Just from watching Allen's movies, I rather doubt he would have any interest in molesting a 7 year-old. He seems much more preoccupied with older teens (like Soon-Yi was at that very time), and I think true pedophiles who are attracted to pre-pubescent girls are not interested in 19 year olds.
OTOH, Paris is nice and I hear Roman Polanski is looking for a roommate.