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The gain patterns determine her weight distribution even when she's slender, so there will be hints when you use the 'Check her out' option on the initial meeting, but it really starts to show the more weight she puts on. 

If you add the girl to your contact list you can see if she has the pattern from there. Also, as these are optional states enabled when creating a new town, you won't find girls with these patterns in existing towns from before the update (unless you use the Create-a-Girl tool).