The claims are being made by refugees and people in Syria willing to speak up about it.
So there are no proofs, only accusations by the rebels.

The thousands of personal accounts from women claiming to have been raped are lies, just as are the accounts of men who served in the Red Army claiming to have witnessed it.
LOL. Where are those "thousands of accounts"?
In two major Berlin hospitals in 1945 there were registered only 9 cases of rape (claimed by the women), by unspecified men.
The statistics of incidents shows that in American and British occupation zones after the war the rate of rapes by military was higher than in Soviet zone (still far from thousands though).