The difference in obviously earth-made vessels and vessels made using the technology of alien races is obvious in construction and appearance. While man-made ships have hard lines, visible riveting, and external plumbing systems, ships made with the construction knowledge of advanced beings are smooth and rounded without visible signs of the construction process. It could be that with the information learned from collecting alien tech recovered on Earth, scientists have been successful in recreating the engineering feats of our extraterrestrial neighbors.
It is certain that there are things not easily explained navigating the skies around the world. Perhaps mankind is further along in their development of airspace travel than those in power would like us to believe. Whether the base for alien technology research is at the popular Area 51 or nestled within a secluded mountain range in rural Nevada, those with inquiring minds are keeping their watchful eyes open for signs of the truth.