Formal:
* Êtes-vous ennuyé(e) ? (Are you bored?) - This is the most formal way to ask.
Informal:
* Tu t'ennuies ? (Are you bored?) - This is the most common way to ask someone you know well.
* T'ennuies ? (Are you bored?) - This is a very informal way to ask, often used among friends.
* Tu t'ennuies pas ? (Aren't you bored?) - This is also informal and suggests you're asking because you think they might be bored.
Example:
"J'attends le bus, je m'ennuie un peu. Tu t'ennuies ?" (I'm waiting for the bus, I'm a little bored. Are you bored?)
Remember to adjust the level of formality depending on the context and who you are talking to.