The Traveller who joins the imaam midway through the prayer and is unsure whether to complete the prayer or not

Sheikh 'Uthaymeen rahimahullah was asked concerning a traveller who joins the imam in the final tashahhud and is unsure whether to complete the prayer in full or to shorten it (because they don't know whether the imam is a traveller or not)? He answered: في هذه الحال ينظر إلى ظاهر الحال، لأن ما ذكرت يحصل في بعض المساجد، يمر بها الإنسان في الطريق ويجد أناساً يصلون، أو في المطار يجد أناساً يصلون، فيشك هل هم مقيمون أم مسافرون , ينظر إلى ظاهر الحال، إذا كان ظاهر هذا الرجل أنه مسافر لكون حقيبته أمامه، وكونه لابس ملابس السفر، فيعتبر مسافراً، وإذا لم يترجح عندك شيء فأتم؛ لأن الأصل الإتمام. "In this case, he considers the apparent situation, because what you mentioned happens in some masjids; a person passes by on the road and finds people praying, or he finds people praying at the airport, so he doubts whether they are residents or travellers. The apparent situation is taken into consideration; if it is appears that the person is a traveller because his luggage is in front of him and he is dressed like a traveller, then he is considered a traveller. But if it is unclear to you, then complete the prayer, because the original ruling is to complete the prayers." [Liqaa Al-Baab Al-Maftooh]