How to properly distinguish kestrel vs peregrine in flight/soaring?

Hello, 

Recently I keep seeing what I think are soaring peregrines, but I cannot properly distinguish them. They are definitely either kestrels or peregrines, however most of my books/online don’t help much to distinguish them. Also, I am pretty poor at taking photos of high flying birds, so I thought it’s more beneficial if I identified them (experience anyway). Is there any tips or tricks to differentiating these birds in flight? (I know what they look like when perched or travelling but more specifically when they soar). Thanks, Ellis