How to ask for feedback from research students on my supervision abilities?
Set the Stage

At the start of each relationship, let each student know that you value feedback, that you are adaptable/flexible in your approach to supervision, and that you will be asking for their feedback both during and at the end of the end of the supervision period.

Reinforce the Message and Values

During the supervision period, look for opportunities to demonstrate your adaptability, your interest in feedback, and it is OK for them to offer feedback even if they might feel a bit uncomfortable.  (Some people resist giving feedback to supervisors, including for gender/age/cultural reasons. They need to experience the process working successfully to overcome that resistance.)

Asking for Feedback

Aim for a face-to-face meeting, though it's fine if some people prefer to give feedback in writing.  Before you ask for feedback, first ask what is important to them in supervision relationship. You need to understand their frame of reference and values. Then ask: "What worked well for you? And what didn't?"  Then you can ask the feedback question: "For those things that didn't work so well for you, what would have worked better for you?"  You are asking how things could be different for them, within their frame of reference.  You aren't asking them to step into your shoes and advise you on how to be a better supervisor.