How can I determine in practice whether I must issue e-invoice to a customer?

Short answer

Assess whether the customer is acting as a business or taxable person rather than as a final consumer. A company registration number or tax identification number is a practical indicator of a business relationship, but the statutory obligation must still be determined from the status of the parties and the particular supply. The absence of business identifiers will usually indicate a B2C transaction outside the mandatory e-invoice regime.