There are two aspects to whether a person is competent to train others on a practical activity.

The first is whether they themselves are competent in the activity.  This is best demonstrated by consistent success in carrying out the activity correctly.

The second is whether they have the ability to deliver the training successfully.  This ability is made up of skills such as communication skills and other characteristics such as being conscientious and patient.

To demonstrate competence in a practical activity it can help to have a checklist of what makes up the competence, such as, selecting the correct bit, placing the guard correctly, whatever all the critical steps are in the activity.  Then the trainer can sign off each of these as and when the trainee is able to demonstrate them.  This checklist is an aid to the trainer as well as documenting the demonstration of competence and this approach is more useful for more complex activities.  For a workshop it might be more proportionate to sign off per machine.

I've sometimes seen skills matrices in which each operative is given a rating for competence in each activity.  These ratings begin with something like "basic - can do routine jobs under supervision" and go up to "advanced - can train others".  This documents that you have considered the competence of someone to train but in the end, that always has to be based on a judgement.

Everyone learns from someone. No one is ever born 100% competent in anything.

Your management need to step up and make decisions:

Q) Do we have staff we believe competent to undertake site tasks?

A) If they are on full rate rather than apprentice / training / new-starter money then the answer must be yes.

Q) Of those we believe competent operatives are any capable by aptitude or with training suitable of passing on their skills and knowledge?

A) If a course or previous experience demonstrates such capability assign them as trainers. If not consider external provision.

Q) Of those trained internally is their use of the equipment satisfactory?

A) If yes the trainer must be competent. If no (e.g. increased accidents) there is a gap to address.

In the latter case you either defer to an external provider or refresh and strengthen the internal capability.

You can then generate myriads of "certificates" to file on their personnel records to wave under the nose of any investigating solictitor or merely update a training matrix.

Ultimately its not down to "competence" but more correctly what instruction, training and supervision the new employees receive.

The real task is documenting what is "learned" sitting with Nelly - this is where any accident investigation should be focused.

The harder task is documenting the person specification to be used in recruitment (the prior skills, knowledge and experience). "Machine Operator" really doesn't say that much - it could be a simple turnstile or a massive earth moving machine.