A damp towel gives good evidence that someone has been sweating. It does not identify which substances have left the body in meaningful amounts. This distinction is easy to miss when a photograph of steam is captioned with the single word detox.
Swedana refers to procedures intended to induce sweating. A historical description may discuss the material warmed or the part of the body exposed. For a visitor, more immediate questions concern the heat, the means of leaving and how a practitioner would respond if the person became faint. A person feeling unwell should not be encouraged to endure discomfort as proof of success.
When you read a claim that a treatment removes a toxin, ask which substance was measured, in what sample, and against what comparison. Without those details a change in the towel is simply a change in the towel. This is not a procedure to recreate by enclosing yourself in an improvised steam chamber.
Further reading: Panchakarma terminology (Ministry of Ayush annual report) · Detox and cleanse claims (NCCIH)