How do you tell the difference between a cyst and a cancerous tumor under the skin?

Skin: Masses under the skin that are rubbery and freely mobile are often benign, but the only way to really know would be an excision or tissue sampling and examination of the tissue by a pathologist. Imaging (such as ultrasound, ct scan, or mri), may also be helpful.

Look, feel, imaging: The look, feel, imaging like ultrasound, MRI or ct scan charecteristics may help. Some remain hard to tell and ultimately may require surgery to confirm. Stability of size overtime is usually a good sign.

Biopsy: A simply biopsy would give you the answer.

Possible bx: A cyst under the skin when palpated gives the suggestion that there might be fluid present. Placing a fine 22 gauge needle into it can define if fluid is present. Most lesions under the skin rarely are cysts but rather lipomas, fibromas , neuromas and the intial growth of desmoid like lesions. Here a biopsy or excision can resolve the problem.