Is it obligatory to ascertain how Hajj and Umrah tour operators obtain visas if there is a suspicion that this is done through bribery?

The ruling on paying bribes to obtain Hajj and Umrah visas, and when it is permissible and when it is not, has been detailed previously. You are not required to verify whether a person deals in bribery as long as it is merely a suspicion; indeed, it is not legislated for you to do so, because the default for a Muslim is soundness and good presumption.