There are somethings that we are all aware of,but we just【just在这儿只有强调的意思,可加可不加】 cannot prove it.
Such as the well-known Mona Lisa which is displayed at The Louvre museum in Paris,we cannot prove that is authentic.
一些我们都知道的东西,但我们都不能证明.如我们都知道卢浮宫有名画蒙娜丽莎的微笑,但我们不能证明挂在卢浮宫里的那个就一定是真迹.
或者：
There are somethings that everyone is aware of ,but we just cannot tell the real from counterfeit/false.
A classic example is the well-known artwork——The Mona Lisa which is displayed at The Louvre museum in Paris,we cannot tell whether it is authentic.
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aware of：知道
tell the real from counterfeit/false：辨别真假.
classic example：典型的例子
authentic：真的（一般形容物品的真伪.）
【P.S：第二个句子里面的tell虽然不是证明的意思,但是它也有辨别的意思.（个人觉得这个好像念起来比较顺?（笑~）
希望以上答案能帮上您.