Would coarctation of the aorta in a teenager be identified when having an ecg is done?

Not really: The EKG might be abnormal as a result of the high blood pressure, but there would be nothing specific to point to coarctation of the aorta on ekg. The blood pressure would be lower in the legs than in the arms and the pulse in the legs weaker than in the arms.

Possibly: If a coarctation is mild or there are many "collateral" vessels (vessels that bypass the narrowing) then the ECG that could show stress or strain on the left ventricle may be normal. An ECG on someone with a coarctation of the aorta may be completely normal.

No not likely: Ct or mr angiography is usual imaging test to detect such.

Possibly: An ECG may pick up on coarctation. But it also may not. Blood pressure measurements are on the best ways to screen for coarctation. Please discuss any specific issues with your doctor if you are concerned.