Some people are comfortable discussing their past, while others need more time. Notice whether they can talk about their ex without making you feel responsible for comforting them, proving that you are different or competing for their attention. Also pay attention to your own boundaries. You can listen with care without asking for every detail or becoming involved in unresolved conflict.
Someone’s past relationship cannot tell you everything about who they are today. Instead of judging them for having an ex or expecting them to speak only positively about the past, notice how they reflect on their experiences, treat other people and behave with you in the present. Those patterns will tell you much more than the mere fact that they were once in love.