Watch the final fight scene of Becoming Part 2, especially the overhead shots. You can tell where the stunt doubles have been used really obviously. Watch out for Buffy's changing hair style and Angel's new sideburns.
I also noticed Buffy's stunt double in the episode where she kills Adam (that weird mock-up demon/human thing), just before she finally kills him and they are fighting, if you look closely at the hair on Buffy, it's a side part, but on the double it's a central one!! I laughed a lot, cuz I've never noticed the stunt doubles before!!!
I also noticed that in the episode called triangle. The troll thinggy knocks over Buffy. But before she has her hair in two pig tails at the side. When she falls to the ground she had a single pony tail.
Sopiha Crawford is on record as having said that Buffy's hair has given her more nightmares than any of the demons she's seen in the show! (SC was Buff's Stunt double in seasons 1-4)
You can tell that it is Sarah Michelle's stunt double throughout the whole fifth season, because she has a different body type. Sarah is more petite than her stunt double, and you can especially notice this in "The Gift", the season five finale, when Buffy jumps off the tower into the portal.
Sarah did "jump off" the tower into the mystical energy. She had wires and it was computer tricks. They wouldn't have used a double for something that simple.
Spike's Hair, it is always Slick right back, yes? watch every fight he has, stunt double has soft fluffy looking hair, not slick greasy looking. thats Every Fight!!! See for yourself.