To check to see if your radiator is clogged open the radiator cap and look down into the radiator and then start the engine and look to see if the radiator is allowing the coolant to flow properly through the radiator.
If the coolant is not going down in the radiator or doesn't seem to be flowing then your radiator could be clogged or your water pump may not be working properly or you have some other part of the cooling system clogged.
Another quick and easy way to check for a clogged radiator is to ask someone to rev the engine while you are squeezing the radiator hose.
If the radiator hose stiffens up and is difficult to squeeze as the speed of the engine increases, this means that the radiator is clogged.
You can flush your coolant system out and radiator by removing the top and bottom radiator hoses and then running water through the radiator through a garden hose.
Or you can use a coolant system flush kit to attach to your garden hose and then flush the coolant system out that way.
Another option is to take the car or the radiator to a radiator shop and have the radiator shop remove and flush and clean the radiator.
Or just replace the radiator with a new one which is what I would do if the radiator is 10 years old or older because it could begin corroding.