Low T is generally diagnosed using total testosterone, but there’s evidence that free testosterone can also be used.
As a part of the European Male Aging Study, researchers compared the symptoms of two groups of men between the ages of 40 and 79. The first had low total testosterone (below 300 ng/dL), but normal free testosterone. The second had normal total testosterone, but low calculated free testosterone (below 6.3 ng/dL).
After considering factors such as BMI, age, and other comorbidities, it was found that the group with low free testosterone showed more symptoms of hypogonadism than those with low total testosterone.
The researchers concluded that free testosterone provides another method for diagnosing hypogonadism in men who might have otherwise been missed by only considering the total value. 
What Counts as Low Free Testosterone?
There isn’t a definitive answer as to exactly what counts as low free testosterone, but the following:
For a direct measurement by equilibrium dialysis, the Endocrine Society recommends using 5 ng/dL.  Another recommendation is to the use lower bound of the laboratory’s range, which will typically be between 4 and 9 ng/dL.
The alternative is to calculate free testosterone using total testosterone, SHBG, and albumin. This can be done using an online tool, such as the one found at:
The ISA, ISSAM, EAU, EAA and ASA recommend using 6.5 ng/dL as a lower bound of calculated free testosterone.  A 2011 review in the American Journal of Medicine suggests 8 ng/dL as a cut-off. And a 2014 synthesis of expert opinions range from 6.5 ng/dL to 7.2 ng/dL.