[header] How to tell if someone has been using marijuana [title] Look for bloodshot eyes. [step] Someone who has been using marijuana may have very red or bloodshot eyes. However, don't rely on this symptom alone as an indication of marijuana use. 
Red eyes can also be caused by any number of other things, including : [substeps] Allergies illness (such as a common cold) lack of sleep recent crying irritants in the eyes excessive sun exposure [title] Watch for signs of dizziness. [step] Someone who has recently consumed marijuana may become dizzy or uncoordinated.