How to ascertain Rahu Ketu nature for a horoscope

[ESP]

Dear Veterans,

Please explain how to find out whether Rahu and Ketu are Malefic or Benefic for a native.
For which Lagnas Rahu and KEtu are malefics and benefics?

On what basis does one prescribe Gomed and Vaidurya ?

I read in Pt. Sanjay Rath's Book "Vedic Remedies in Astrology" the following example.

"If a person's Nakshatra is Satabhisha, the lord is Rahu. The native can wear Gomed in the LEft hand if Rahu is MAlefic for the lagna and right hand if he is benefic"

Please explain

[ESP]

Dear Veterans,

Any one of you please throw light no the topic as not able to find the info anywhere