As others wrote, you don’t know except for the saints the Church recognizes. However, your prayers are still effective because purgatory, like heaven, exists outside of time. Even if your loved ones are in heaven, your prayers can somehow aid them back when they were in purgatory. This also applies to people we might think are in hell. Even if they died 20 years ago, our prayers can aid them just as they are dying and meeting Jesus as if their death just happened.