Sonic R2R Tape Speed Tester available?

Where can I locate a reel to reel tape where I can test my Pioneer RT 707 for playing speed accuracy at 3 3/4 and 7 1/2 ips playing speeds? My unit has a variable switch for adjusting the pitch of the tape speed but I want to make sure the unit's standard speed is on the mark before I start adjusting it. Essentially, I'm asking for the equivalent of a strob for a turntable to adjust the speed to the required standard, but for my reel to reel. Any resources or ideas will be greatly appreciated. Go analog!
Just a thought. If you have any prerecorded tapes that indicate the timing of individual tracks, play them with a timer in hand (any watch with a second hand) and see how close they are to the indicated time -- fast or slow. Then adjust the pitch control as necessary. Yeah, it's time-consuming and primitive, but at least it doesn't cost anything.
find used electronic equipments from surplus store a frequency counter and a audio signal generator . give the recorder's line in-put in 1K hz signal and do recording at the same time connect the frequency counter to the out- put to check or adjust the speed to mach to 1K hz, this is the best and cheaper way to do, a test tape will cost more than the used equipments
sorry I made a mistake use another recorder to record and play back to your and adjust