Had a ct done for chest pain to check for blood clots, would they have seen if i had lung cancer or something if they weren't looking for it in the er?

Doubtful: No- if nothing was noted, in all probability it is not cancer.

Yes: most other obvious problems would also be seen in the chest at that time.

Most likely,...: Assuming you had a CT of the Chest, that would reveal any masses in the Chest area imaged, that may be suspicious and warrant further investigation.

Yes: The final reading would be done by radiologist, CT of chesst not only detect pulmonary embolism, but tumors, infection, rib fractures, excess fluid in lungs and areas of decreased ventilation.