Answered by Shaykh Faraz Rabbani
Should we join the prayer as a masbuq if we do not know if the Imam is praying 2 or 4 rakas i.e. muqim or musafir? This question arises often at Islamic functions where one does not know who the Imam is and joins the prayer after the takbir tahrima and misses any announcements from the Imam regarding his status.
What does one do if one cannot work out if one has missed 1 rakah (therefore praying behind a musafir) or 3 rakas (praying behind a muqim?)
Also, I was told that if I lead the prayer as an Imam when I am a musafir, after the salams I should announce to the congregation that I have prayed 2 rakahs of a musafir. Is this true, and if so does it hold the answer to my first question ? Do I just say “we prayed the traveller’s prayer”?

Walaikum assalam,
If you know that the imam is praying the fard prayer, you can assume that he is not a traveller, because that is the default assumption.
If you lead others while a traveller, you should inform people before you start to pray, and then clearly after you give your two salams, so people act accordingly.
Wassalam,
Faraz