Orsdxy

According to https://www.atlasobscura.com/places/uscanada-border-slash, the US-Canada border slash through the trees can be seen from space.

You did not specify if you are thinking of any border or only an anthropogenic one.

Anthropogenic (they will likely be visible due to different levels of economic development):

1. Egypt and Israel (different level of development visible on both sides)




2. Haiti and Dominican Republic (less vegetation in Haiti)

Natural ones (rivers and mountain ranges can be seen from space):

1. Mexico and USA (Rio Grande)




2. Germany and France (Rhine)




3. Germany and Poland (Oder)

While LED lighting is taking over (and is likely CW), plenty of outdoor street lights in cities and highways still use high pressure mercury and sodium discharge lamps running on mains AC voltage. They don't rectify and so will produce two pulses per cycle of AC.

Therefore any technique that can chop up time, a rolling shutter effect or simple aliasing in time from a video device, or even a spinning piece of paper with slits on the end of a space-rated paperclip would be able to distinguish a 50Hz country from a 60Hz country.

So Suriname|French Guyana or Argentena+Chile+Uraguay+Paraguay+Boliva|north thereof or Saudi Arabia|Neighbors or perhaps Liberia|Neighbors (needs better data)

For even more fun, just have a look within Japan, Tokyo prefecture and north is 50 Hz, and south of it is 60 Hz!

Source

Two screen shots from the YouTube video 50 Hz vs 60 Hz vs 400 hz A.C. Hum Sound Comparsion converted into GIF:

click/open separately to view full size

Belgium, sort of:

Belgium has road lighting on almost all of its roads. The neighboring countries don't. The southern Netherlands, the Ruhrgebiet and the Lille area are densely populated so the distinction is less clear here.

A part of the border between Austria, Switzerland and Germany may be seen from orbit.

For the biggest part of the Lake Constance, the Upper Lake, the location of the borders within the water is not defined. Therefore the lake itself is the border, a very wide one, easily to be seen from orbit at day and night.

Within a smaller part, the Lower Lake, the border within the water is defined.

See Wikipedia for International borders of Lake Constance or this chart of the Bodensee.

Another part of the border between France and Germany is the river Rhein between the towns Basel and Karlsruhe, see this chart.

Looking at this high resolution image of the Earth at night, here are a few borders I could find.

US/ Mexico can be partially seen, particularly in the Western US.

Guatamala and its southern neighbors of Honduras/ El Salvador

India/ Pakistan is VERY noticeable.

North/ South Korea is also VERY obvious.

Italy seems pretty obvious, but it is probably because the northern end of the country isn't very habitable.

Another border which is visible due to the lack of light is the Haitian border with the Dominican Republic:

The area highlighted on the left is Haiti, of which only Port Au Prince can be seen at night. The bright island on the right is Puerto Rico.