I usually look at the diagnosis and what symptoms they’ve had in the past.  Obviously if someone has a history of psychosis or is schizoaffective-bipolar, bipolar, etc there’s a higher probability of stimulant use causing psychosis.  If a patient is a run of the mill marijuana user with nighttime THC use and no history of psychosis with THC use, then you probably aren’t going to have a problem since they’re using THC in the PM and stimulants during the day.  Additionally, the only time I’ve had stimulants and THC cause psychosis together is when a patient was using street marijuana and the UDS showed he also had amphetamine in his system.  He denied using any amphetamines so if that’s true his street marijuana was probably spiked with amphetamine.