How to measure the response time from the keyboard?

Question

Hello,

I have a winform system showing photos, the system measure the elapsed time between showing the photo until the user pressed any key button on the keyboard with a stopwatch.

I saved the time in a Dictionary<image, long> in such a way that for each photo i have the response time(saved in miliseconds) from the user.

How do i verify that the times that the user pressed is the real times saved in the dictionary?

Is there any tool in VS2012 or other, to help me Verify it?

Thanks.