You can if you're trained to do so, but that only give a general idea about how good the weld is. Like 65 said, the real quality of the weld is hidden and can only be inspected with x-ray machines or destructive testing. When a welder is tested and certified, they make a weld that is cut up, bent, etched and internally inspected. If they pass the test, an inspector will generally know that the welder is able to make a quality weld and will only have to visually inspect the welds on a project. Inspectors have been known to make a company grind out the welds made by a non-certified welder and have a certified welder do the job.
Joints are typically sized per wall of the tubing, so a large weld on a thin wall tube is not necessarily a good thing. It can weaken the tubing and make it brittle which if it breaks will do so just outside the weld.