Depends on where you are. Here they can use visual estimation. If they think you are doing 40kph over the limit, your vehicle goes bye-bye.
Here's a quick story:
Buddy was riding his sportbike w/ his girl on the back on a local highway. Another similarly looking bike was going faster and doing some douche-like moves, also with a passenger. Someone calls the other bike in, and the cops stop my buddy. They don't clock him speeding, they don't get a speed estimate - nothing. They stop him purely on the fact of him matching the description of a civilian's phone call. The result was $600 worth of immediate fines, bike towed for a week, and license lost for two weeks. In a week those officers showed up at his house and took his license for another three months.
He paid $4000 for a lawyer to send a strongly worded letter (which got him his license back after a month) and who will also join him in court to fight these tickets. Once the trial takes place and he (no doubt) wins, he'll be proceeding to sue for costs.
All this because some grandma called 911 on her cellphone and said that a red motorcycle with a passenger was speeding.
And I know my buddy real well, and know how he rides (we ride together all the time) - this story isn't made up. I picked him and his girl up when the bike got taken. Welcome to beautiful British Columbia.