How do you tell the difference between a Hair Fair vs Hair happenin's Barbie?

[2010byandsel100]

I know Hair Fairs were only heads but most were sold in the secondary market on a body (supposedly tnt, bend leg) and the hair happenin's had the white / red dress. 

but, short of having a head in the package or the hair happenin's in a box,  what distinguishes them from each other?  

[tree*fern]

@2010byandsel100  Barbie Hair Happenin's (1971) does not have bangs, and Hair Fair heads have side-swept bangs.

Hair Happenin's came only with centered eyes and titian (red) hair. Only the 1971 -74 Hair Fairs had both centered eyes and titian, blonde, or brunette hair. The 1967-70 Hair Fairs had side-glancing eyes and blonde or brunette hair.

There's a good close-up of Hair Happenin's on this reference page:
https://www.myvintagebarbies.com/hairhappenins.htm