star wars - How did Mace Windu conclude that Darth Maul was a Sith?

At the very end of Star Wars Episode 1: The Phantom Menace movie, Mace Windu and Yoda had this conversation:

Mace Windu: There is no doubt. The mysterious warrior was a Sith.

Yoda: Always two there are....no more...no less. A master and an apprentice.

As I think, a Force-sensitive guy using a red lightsaber attacking a Jedi couldn't necessarily be a part of Sith order established by Darth Bane. For example, Darth Maul wasn't a Sith during the events of Star Wars: The Clone Wars TV series which was clearly stated by Palpatine.

How did Mace Windu conclude that the mysterious warrior was a Sith?