It depends whether they say they are agnostic or they deny that.If they say they are agnostic, we need to talk to them and present the clear evidence that Allah is the Creator of everything.Usually such people claim that they use their intellect, hence we need to focus on intellectual evidence to prove from Quran and the sayings of the Prophet (SAWA) and Ahlul Bayt (AS) who have the maximum intellect that the faith in Tawheed is the compulsory outcome of the proper thinking.
Wassalaam