They discovered it because one of the perpetraitors, Francis Tresham, feels compelled to inform his brother in law, Lord Monteagle, that he should not attend the opening ceremony. Monteagle passed the letter to the authorities , so that the guards stopped Guy fawkes from blowing up parliament.
Lord Monteagle was born in 1575 and died 1st of January 1622, so he was 47, which means he died young. Francais Tresham died on the 23rd of December 1605.
We now celebrate bonfire night on the 5th of November with fireworks because they're made out of gun powder and we light them. Also we get scarecrows and light a bonfire and put the scarecrows in the bonfire. We do this because they put Guy Fawkes in a fire.