Well, you can can put masks on dummies and see what fraction of aerosols of different sizes they stop, you can interview people after an outbreak and find out w

Well, you can can put masks on dummies and see what fraction of aerosols of different sizes they stop, you can interview people after an outbreak and find out what protective measure they took and whether they get sick. You can look at which regions introduced mask mandates when and see what statistical result that seems to give on the number of covid cases. There have been an awful lot of studies on this topic over the course of the last 3/4 year and I'm also taking into account a study the WHO did after SARS-1 on mask effectiveness in Beijing. I wouldn't neccesarily have thought beforehand that fraction of particles stopped would line up neatly with reduction in chance of getting the disease but it seems to, at least roughly. EDIT: Oh, and there are animal experiments where they put a layer of mask material between hamster cages and see if it prevents infections in the other cage.