Can you tell if there's s blockage in the heart with a ekg ?

Yes: An ECG is an interpretation of the electrical activity of the heart. Due to damage of the conductive tissue that carries electrical signals, in a heart attack or ischemic event due to coronary blockage, changes will occur in specific areas of the heart. Each part of the heart is supply by a specific coronary artery. For an anatomical correlation, a cardiac catheterization will have to be performed.

Possibly.: An ECG may be entirely normal, even with severe blockages. An ECG can suggest blockage with changes that occur when the heart is actively receiving reduced blood flow, and will often show damage that has already occurred. Observing the ECG during exercise (stress test), particularly if coupled with an image such as echo or nuclear scan, will be more helpful in screening for blockages.