How do I know if I smoked laced weed?

You may not. Marijuana comes from the cannabis sativa plant and& alters the mind through thc. Thc concentrations vary depending on what part of the plant is used. You never know if thc has been mixed with another chemical. Contamination of the leaves with pesticides & other chemicals is common. The strength varies greatly from plant to plant and batch to batch. You never really know what you are getting.