The most foolproof way for this to be done is to get a search warrant signed by a judge. In other words, what is required is a search warrant. Given that, we can ask what is required to get a search warrant. In order to get a search warrant, the police must show that they have probable cause to believe that there is marijuana in the house. Probable cause is not something that can be easily and exactly defined. It is a level of proof that is more than a simple suspicion or hunch but less than a complete certainty.
So, for example, police would not have probable cause to search a house for marijuana simply because an anonymous phone call alleged that the house was being used to grow it. But the police might have probable cause if there was an anonymous phone call and people had been observed bringing in equipment that could be used for growing a large number of plants indoors.
Thus, we can say that probable cause is what is needed in order to get a warrant to search a house for marijuana.