What To Do
Assess the horse's general health using the Whole Horse Exam (WHE), paying particular attention to the feel of the swelling, and the presence of heat or pain. Assess for lameness at the walk and pay attention to the stride length at the walk. Take a photo, trying hard to have the horse bearing weight when you do.
When evaluating an apparently swollen stifle area, always try to get the horse standing squarely behind. If the horse is very lame and will not bear weight on the limb, you cannot assess the stifle area well. You should be cautious of assuming that the area is really swollen. Always compare the look and feel of the apparently abnormal stifle area to the opposite side.
Share your findings and concerns with your vet.
What Not To Do
If a horse is lame and not bearing weight, be very careful of assessing the stifle area. It is commonly (and often incorrectly) interpreted as swollen (or abnormal) in horses that are non-weight bearing.