Methods for determining the fouling propensity of a hydrocarbon stream and for reducing fouling are provided. In one method, the fouling propensity of a hydrocarbon stream is determined by obtaining a parameter indicative of the fouling propensity by a regression of a series of temperature measurements data for the hydrocarbon stream exiting a test unit. In another method, an effective minimal amount of an antifoulant is added to a hydrocarbon stream to reduce fouling, where the amount of the antifoulant is determined based on the fouling propensity of the hydrocarbon stream.