There is very low lot water pressure in all my hot water faucets. I have an oil-fired boiler and have been told the problem may be in a "coil" on the boiler. How can I determine the cause of he lack of pressure?

So your cold water pressure sounds like it is fine, right. You first need to confirm the cold pressure going to the boiler, it should be th same as the cold pressure elsewhere. If the pressure in is fine but out is terrible, you have narrowed the field. I would shut down the fuel and the water and then open the connection where the hot water exits the bolier, inspect for build up.