What are the signs of soft tissue sarcoma?

Painless mass. Most soft tissue sarcomas, with few exceptions like synovial sarcoma, are painless. Patients and even some doctors are falsely reassured by the absence of pain. Sarcomas may grow quickly or very slowly, but any painless mass has to be evaluated appropriately. While a painless mass can turn out to be a benign lipoma, a sarcoma has to be ruled out. An MRI can readily diagnose a lipoma.
A mass. These always announce themselves as a mass. You may feel it, or it may push on something. Very rarely do they produce remote / paraneoplastic effects. They are a hugely variable family and i would urge you to have several pathologists examine the tissue prior to a definitive diagnosis being made.