Xerox's formal training program PSS1 improved with age (PSS2 & PSS3). Ultimately, these evolved into SPIN (Situation, Problems, Implications, Needs-Payoff).
These were ultimately founded in the SR's ability "to ask questions, the answers to which would advance the cycle". So, out-of-the-gates, you need to rely heavily on your ability/willingness to listen to your suspects.
These training courses built on objection handling and emphasized rephrasing the suspect's comments (to prove you were listening, to ensure you understood, and to buy you some time).
All of this to say, it isn't a question of "questioning skills" per se, rather, it's understanding where/when ...
I'd seek out Huthwaite (who acquired rights to XLS) and see where they are with sales basics. OR, find a local training company which stresses these fundamentals. OR, find an ex-Xeriod who still has their sales training manuals!
Good luck & good selling!
Pat