All pet attack speeds were standardized to 2.0 in Burning Crusade Classic, so they should no longer be a factor when choosing a pet. Please let us know if you find any that differ!
I use the following macro, which you can either type into your chat window, or create a macro button with it (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.