General description:
* Comment sont-ils? (Formal) - This is the most common and literal translation.
* Quels sont-ils? (Informal) - This is a more casual way to ask the same question.
* C’est comment, eux? (Very informal) - This is a slangy way to ask the question and is only appropriate with close friends.
Specific characteristics:
* Quels sont leurs caractères? (What are their characters?)
* Comment sont-ils en tant que personnes? (How are they as people?)
* Comment sont-ils à vivre avec? (How are they to live with?)
* Comment sont-ils dans le travail? (How are they at work?)
The best way to say "what are they like" in French depends on the specific situation and the level of formality you are aiming for.