It may be pre marked you just will have to look it up in the books that are from the 50s and back to beginning of Fenton. I have over 30 Fenton Vases and cherish each one. I have some milk glass fenton compotes and candy dishes that are before they were marked but I know they are Fenton for I bought them at the factory in WV for I lived 10 miles from that factory. Also have many other pieces which are fenton but not marked. Back in the earlier days people did not leave tags on their purchases. That began later on in the many years they were open to the public. I think it was the late 50s when they came out with a Fenton mark.
You say it is crystal clear but it looks green on here. You might need to take a photo where it would turn out to look clear. But they did not make much that was clear.