How does one join a congregational prayer that is performing Jam' Taqdeem (combining Dhuhr and Asr prayers, or Maghrib and Isha prayers, by praying them at the time of the earlier prayer), if one does not know whether they are still praying Maghrib or have already begun praying Isha, and what should one intend in this situation?

The first option for someone who enters the mosque and has not prayed Maghrib, and finds a congregation praying, is to start praying Maghrib alone. Then, he prays Isha with them if he catches it. It is also permissible for him to join the congregation with the intention of praying Maghrib. If it becomes clear that the congregation is praying Isha, the worshipper should sit for the tashahhud after the third rak'ah and perform the taslim, either alone or with the Imam.