My experience flying from the US to France through Portugal was that an agent at the gate looks over whatever paper you give them and highlights "PCR", "NEGATIV

My experience flying from the US to France through Portugal was that an agent at the gate looks over whatever paper you give them and highlights "PCR", "NEGATIVE" and the date and then hands it back. Then in Portugal they asked for the paper and waved me through, and the same at the French border. So logistically, pretty easy to fake (unless you're stupid enough to be trying to buy the fakes in the airport like some guys who got caught in Paris a few months ago) but I agree with other commenters it's better than nothing and will almost surely improve the rates of testing/precaution around travel.