The strength gauge was a small spherical device used by the Tok'ra when they wished to assess the strength of an individual.
The ball shaped machine was given to a person who simply needed to squeeze it with all their strength. The ball would then emit a high pitched noise, which got louder the more pressure was put on it, sending information wireless to an accompanying display device, which provided the details on the strength used on it. The scientist could be a short distance away from the test subject.
When Colonel Jack O'Neill was given an Atanik armband, he was given a strength gauge by the Tok'ra scientist Anise. The first time he was tested, Anise noted his strength was five times the strength of a normal Human; later, it was sufficient enough to break under the strain of the Human's enhanced strength. (SG1: "Upgrades")