How to sex albino corys

[Kryten]

I know how to sex a few types of fish but I am not sure how to tell whether my albino corys are male or female. Can anyone leave tips on how to tell, what physical differences to look for

Also, in pet shops, these little fellas seem to live on the bottom doing nothing, in my tank now they whizz about all over the place like fishies on speed. Is this normal?

 

[Macquatic]

Viewed from above, the females are much fatter behind the pectoral fins than males, even in young fish. Also if you look closely the femlae's ventral fins are a rounded shape compared to the male pointed version.
Cories always zoom about a new tank for a few days......so yes, this is normal. Mac.

 

[Kryten]

Sorry to be dumb, but which are the ventral fins