You know that a sith is trying to kill you if…
He/she wants to meet you in a tomb, a temple, or the ruins of a temple or tomb.
He/she wants to meet you in his/her office.
He/she wants to meet you just about anywhere, actually.
He/she is in competition with you for anything.
He/she is helpful in giving you information, advice, or items.
He/she is your master or another apprentice of your master.
He/she is your apprentice.
He/she is a Lord/Darth who is an enemy or ally of your master, or that darth/lord’s apprentice.
He/she comes into contact with you in any way. It’s just a matter of time.