I have a question about checking the fit of my western saddle on my horse?

i have a horse who was originally trained western, but steps and goes like a dressage horse, so was trained dressage too. he still knows western because i have tried it in my english saddle, but we have two old western saddles gathering dust in the back of our tack room and i wanted to know how you check to see if it fits him. help me!!!!!!