The Earth's early atmosphere was formed from gases released by volcanic activity. Nobody was there to measure it, so we say it is thought to have contained certain gases. Scientists work this out from evidence such as very old rocks and from models of how the young Earth behaved.
The early atmosphere is thought to have contained little or no oxygen. It had a large amount of carbon dioxide and a lot of water vapour. There were also small amounts of other gases, such as nitrogen, methane and ammonia.
This is very different from today's air, which has about 21% oxygen. When you are given a table or chart of gas percentages, compare the oxygen and carbon dioxide values to decide whether it describes the early atmosphere or the atmosphere today.