There are a few great apps to test your iPad or iPhone Internet speed which you may want to download but all speed tests ultimately do the same thing, and that is to measure the upload and download speeds from wherever you are at the time. Speed test accuracy will not vary significantly from 1 app/website to the next.
Knowing what your upload and download speed is helpful if you need to transfer any file over 1MB.
Don't get stuck having to wait for a file transfer to complete by only performing the transfer when your connection speed is good.
Breaking a connection that resumes later uses additional data, so be sure of your speed before you start.
iPads and iPhones are very efficient at data usage which limits wasted data but always check your connection speed before transferring a file.