The way to demonstrate your competence is to keep records of your performance data.
For colonoscopy this would include your key performance indicators such as caecal intubation rates, sedation rates, comfort scores etc. You should also keep a record of any complications or adverse events and engage with your local endoscopy audit process.
It is good practice for a new employer to assess your endoscopic skills before you start to practise independently in a new unit. JAG certification is not a mandatory requirement for experienced endoscopists. This guidance also applies to endoscopists who have trained abroad or became independent before the new JAG certification process was in place.