The short answer is no. In my experience a chest x ray does not show all problems but is a good start. In fact I have seen cases of lung masses and lung disease not clearly appear. I have seen aneurysms of the aorta not show up. Fluid around the heart and infections of the lungs be hidden. There are also life threatening conditions that a chest x ray has no ability to identify like clots to the lungs or dissection of the aorta.
The reason for this is because the abnormality may be small or hiding behind another structure like the heart. Sometimes it is hard to see and the radiologist may not report it. Sometimes the technique may not be the best. Being a large patient may reduce the radiologist’s ability to see an abnormality. If you do not bring in your old x rays, an abnormality may be not be seen or misdiagnosed as something concerning. It is important that the radiologist reading your x ray has all your history and symptoms to get the most accurate diagnosis.
I think that if you have symptoms referable to your chest, like a chronic cough or chest pain, and your chest x ray is normal, I would not necessarily assume all is truly normal. I would discuss your symptoms with your provider and see what other testing is appropriate. This may include a Cat scan of the chest. This test is much better at detecting many problems like infections, cancer, clots, fluid and so on. So a normal chest x ray, unfortunately, does not exclude many problems of the chest..