Sterling silver is usually stated in an abbreviated way or expressed as 925 (parts silver per thousand) or similar, somewhere on the item. "TG.S" doesn't seem to have any obvious meaning and the difference in appearance of the front of the pendant from the rear suggests a plated item to me. The cheap, plated chain link attached to the pendant is also an indication of costume jewellery.I think the Scorpio pendant is sterling silver. The rear is stamped "TG.S". It does not look like any letters are worn away before or after those three characters.
Your detector should be able to test the find electronically, as sterling silver is extremely electrically conductive. If you have a known silver item, compare its reading on the detector's target meter with the reading for the pendant. The readings should be very close, but if the pendant reads lower, it probably isn't silver.