As Rich said, the Babolat RDC is used to do this, but it isn't measuring the actual tension on the strings. It's measuring the force it takes to deflect the strings in the middle of the string bed by about 1/2." Recently, when I strung a Prince Graphite Michael Chang OS at 60 lbs. with a nat gut/syn gut hybrid the RDC rating was over 70 right after the job was finished. The RDC can also be used to measure total racquet weight, swingweight and frame flex. Laserfibre also makes a device called an ERT700. It also measures the force required to deflect the string a preset distance to determine stringbed stiffness.