While buying a painting, you can ask the author it to print a certificate for the picture.the artist, you can ask him to make a certificate for a picture (free of charge).
Samples of certificates:
You can independently order a professional expert attribution view for artworks.
For modern paintings attribution costs 80$ and should be paid directly to an art-expert.
To order attribution view send a message to an art critic Dashkova Marina.
Samples of art expert attribution: