The give away is the length!!
Because of the lack of SATA to USB adaptor, native USB drive are almost certainly Shorter!!!
Here's a length comparison:
Seagate Freeagent Goflex is a SATA drive, about 5" or 13cm long .
WD Elements SE 1TB USB3.0 is Native USB is less than 4.5" or 11cm long
WD Elements 320G Drive is 5" long or 13cm long is a SATA drive
Here's a picture of Native USB only drive showing their relative length.
Here's a picture of the internals explaining the differences in length.
SATA drives has the adaptor
USB drives no adaptor hence shorter.
You are also able to tell by the external box as it shows actual size hard drive and is also a give away.