وَسْـَٔلْ مَنْ أَرْسَلْنَا مِن قَبْلِكَ مِن رُّسُلِنَآ أَجَعَلْنَا مِن دُونِ ٱلرَّحْمَـٰنِ ءَالِهَةً يُعْبَدُونَ Qur’an Az-Zukhruf (43:45)Wa i sal man arsaln a min qablika min rusulin a ajaAAaln a min dooni a l rra h m a ni a lihatan yuAAbadoon a
I.e.. "look into the earlier revelations and ask thyself".
That is, by examining their Message, and asking the learned among their real followers. It will be found that no Religion really teaches the worship of other than Allah.