Terraformed Mars in the 19th Century

[Rynn]

Well I'm pretty sure the answer to your question is "not a lot". It wasn't until the 20th century that images of Mars good enough not to feature significant artefacts (like the canals) were possible. These two images are maps drawn of Mars by late 19th century astronomers and as you can see they only faithfully document very large features (at best). Given that the Martians are said to be comparable to Earth at the time we can assume their largest city is also comparable to Earth's; at the time this was London. If you watch this video at timepoint 4:54 it starts to show the Victorian era London. The outline of the city the use is for all of greater London, here's a borough map with scale bars. As you can see greater London is approximately 25 miles across, going back to the video by the late Victorian it looks like the diameter of the developed area is only 10 miles.

Taking all that together I think it's highly unlikely that 19th century telescopes would be able to spot Martian settlements. Even if they were lit up that is a very tiny spot to make out at a time when atmospheric optical illusions couldn't be corrected for.