National boundaries are usually not visible from the air. They are just lines on a map, and you can cross entire continents without knowing which country you are flying over.
Exceptions are the bright halogen lamps on the tightly-guarded border fence in the desert between India and Pakistan which can be clearly seen from an airliner flying overhead at 11,000m. Even from the Space Shuttle, the Koreas appear in sharp contrast: bright on the south and completely dark north of the DMZ.
However, the Nepal-India border is open and is not even noticeable from ground level. One side of a tea shop is in India, and on the other side is in Nepal, where you have to reset your watch 15 minutes ahead. Many visitors who have strayed into India and unknowingly taken pictures with their mobiles have been detained by Indian BSF.
But there are some sections of the Nepal-India border that can be detected from the air, and one such is a straight line in Parsa district where a forested patch of the Balmiki Nature Reserve in India is separated from the rice fields of Nirmal Basti on the Nepal side (see pictures, above).
Google Earth satellite images show the border visible even in 1984, and the steady deforestation on the Nepal side by 2016. The frontier is visible from the left side of planes as they begin their descent into Kathmandu.
The other section of the India-Nepal border that can also be seen on satellite image is in Dang, where the slopes of the Chure Range north of the border are severely denuded.