Early spacesuits (like the SK-1 or the Mark IV) were designed before anyone had gone into space. How did they test that the suits would perform well in a vacuum? Did they send them into space with sensors inside? Did they have large vacuum chambers to put them in?
They were tested in vacuum chambers. They actually still do, it's by far simpler than sending something in Low Earth Orbit (LEO) and beyond to later realize they need to make changes to it. Vacuum chambers also allow for a controlled depressurisation and repressurisation, which is another advantage to having them tested that way. Some interesting related videos: