Attack speeds were standardized for Burning Crusade, so all pets in BC Classic should have a base attack speed of 2.0. But if your pet has been taught the ability Cobra Reflexes then its attack speed will be faster than this.
However, it appears that at least some pets still currently have the wrong base attack speed, and we've received multiple reports of speeds faster than 2.0 - many unlikely to be the result of Cobra Reflexes. If you happen to find any pets like this, please report them in the following Blizzard forum post, or let us know by mail. Thank you!
I use the following macro, which you can either type into your chat window, or add to a macro button (hit ESC, then select 'Macros'):
/run mainSpeed, offSpeed = UnitAttackSpeed("target") print(format("%s: attack speed = %.2f", GetUnitName("target"), mainSpeed))
Then you simply target any creature of interest and use the macro.