Terraformed Mars in the 19th Century

[Cray]

If Mars had significant oceans, it would be blue (where it wasn't white from cloud-cover), not iron-oxide red.

Yes. It might almost prompt renaming the planet, like Turtledove did in "World of Difference." (Mars was "Minerva" in that novel, because its seas 'flashed' like the eyes of Minerva.) But that prompts questions about how a non-red Mars would influence human history. At a minimum, the Greek and Roman religions invested a lot in the notion that Mars represented the bloody God of War. But then you end up with an alternate history branching off thousands of years ago rather than a fun steampunk romp.

So, I've kind of been plugging my ears on this point and making loud, "lalalalalala" sounds whenever I think about the impact of a non-red Mars on human history. "Everything turned out normal until Space 1889 happened, dammit!"

If Mars was home to any technological civilization at that level of development, the society would use large swaths of the land as farms--besides of course the natural vegetative cover.

I know agricultural fields are visible from low Earth orbit, but would Martian fields be recognizable as farmlands in the 19th Century from Earth?

Also, they should have been able to detect the high concentrations of oxygen and nitrogen in the atmosphere of this Mars using spectroscopy.

Eventually, yes. However, it wasn't until 1921 that Mars' surface temperatures were measured. In the 1870s, water was supposedly reported in the Martian atmosphere, but those readings proved false. Repeatable spectroscopic measurements don't seem to be made until the 1920s - 1930s, when the water measurements were debunked and oxygen levels were estimated.
http://en.wikipedia.org/wiki/History_of_...parameters