How do you know if a Renoir painting is authentic?

Before proceeding to authenticate any articraft first make a small research your self and find out if there is something that can give it off as a reproduction. If this fails than the next step is to find an art expert. He will make a written or verbal assesment perhaps under a small fee and tell you wether your articraft is genuine or reproduction.

There is a Renoir painting called 'Roses'.

There are on the market plenty of handmade reproductions, made to look original.

The original is in a museum.