It cannot be proven unless there are witnesses or hard evidence such as a photo or film; or something written by the accused that points towards his guilt.
Even if he has a very distinguishing mark in an intimate place that the victims recall seeing, that would only prove they saw him in a state of undress, not when or where.
As always in a criminal trial the jury will have to be satisfied beyond reasonable doubt that he is guilty - the CPS belief that there is enough evidence doesn't count for much, such as in this very recent case:
http://www.dailymail.co.uk/news/article-2336358/Sex-claim-MP-thrown-lesbians-lover-tells-lies.html
There are harrowing events in my own childhood that I remember vividly but I can't remember the year, let alone the date.
I am very glad not to be on the jury in this trial.