So far, I've had great success with the majority of the ones I've signed up for. Most typically advertise themselves either as "guaranteed" (meaning you're allotted X $ value or X amount of tokens for completing their tasks) or as "a chance to win", typically to 1000/10000 random people, sharing a set value. I usually avoid the ones that are "chance to win" / lottery style and aim to only sign up for ones that specify a set reward amount.