Public service employees are required to show identity cards when they come to your home. Examine the card carefully as fake cards have been used. The card should have a photograph and the name of the organisation. If you are at all worried, ring the organisation to check the caller is genuine. Use the telephone number given in the phone book or on your utility bill, rather than the one printed on the identity card. A genuine worker won't mind waiting.
If you are at all nervous, you could ask whoever is at the door to come back at an appointed time and arrange to have someone with you.