Can you reuse filtered oil / How do I know that my filtered oil is clean for use

Yes, Oil that has been filtered can be reused if it comes out clear

If your oil matches the below descriptions then it can be reused:

-Oil is void of debris or particulates
-Oil is not milky or chalky
-Oil has clarity (the color may be yellow or amber and still have clarity)
-Your vacuum pump is achieving a pressure (mTorr) suitable for freeze drying