Can a thymoma be detected via chest x-ray?

Sometimes. If thymoma is not causing symptoms,sometimes identified incidentally,found on chest X-ray or CT scan that is performed for another reason. If symptomatic chest X-rays, CT scans or MRI scans or the combination of PET and CT scans, are performed.While mass in the anterior mediastinum suggestive, definitive diagnosis can only be established when the mass is either removed or biopsied with pathology.
May or may not be. It depends on its size. Since these tumors arise in the mediastinum, chest x-ray will only show advanced tumors. Otherwise ct of the chest is the best test to fully assess the mediastinum for the smallest of tumors.
Yes, if it is large. Thymomas are usually studied by ct scans, but particularly large tumors can show widening of the upper chest(mediastinum) on a chest x-ray.