Well I will offer my three cents. lol
1. my speedo is right on the money, never failed me yet. Even had a local officer clock me to make sure
2. you can always try using a g-tech meter, not that it is always the most reliable but an option
3. do a simple math test going down the highway at 60. Assuming that the mile markers are accurate. Thats what I used to do in my 73 chevelle when the dash lights wouldn't work. if you are doing 60, than one mile is one minute. 75 is 48 seconds, etc.
4. go to your local track and run a quick 1/4 it will tell you what your trap speeds and such are.
5. dont speed?