Authenticating the paintings of Joan Miro can only be done with certainty from research.
Miro spent his life in Spain, pretty much confined to Catalonia and Mallorca, and in France, mostly in Paris. This is where most of the archives and the bulk of the research material about Miro’s production are located.
Joan Miro never lost anything. Miro never ran away from anywhere leaving possessions and art behind. Miro had no debts, no drinking, no messy affairs, no political involvement for him.
If Miro produced a painting, some record of it will be found somewhere. This is why authenticating a Miro is mostly a research proposition.
Our two art historians and researchers in Spain and the other two in Paris, have access to the entire body of reference material about Miro’s production. Miro’s trips to the US are well documented from the US side, where of course we are at the main art libraries every day.