In addition to the Fireman's duty to tend the fire:
1. There were not really many routes with cab signals. Most steam engines operated on routes with wayside block signals, and a large amount of track routes had no signal system at all. Operation there was strictly by timetable and train orders.
2. At known locations, such as an open office of communication on a left hand curve, every Fireman stopped whatever he was doing and looked for the order board, then advised the Engineer by hand sign or by shouting, the color of the board. If orders were to be picked up on the fly, it was normally the Fireman, standing in the gangway, who got them. Even if no orders were to be picked up, the Fireman had to watch station employees as they gave a roll-by inspection from the station platform, and did the same every time they passed a track gang or signal maintainer on the left side.
3. Nearing meeting points, any Fireman with a desire for survival frequently checked the track ahead to be sure that the opposing train had not overrun the meeting point. He had to read the orders when they were received and that was the last he would see of them, so he also had to mentally keep aware of all meets and temporary speed restrictions.
4. When there was a Brakeman in the cab, his duties included looking back at the train on curves, crossing over to the appropriate side gangway. If the Brakeman was stationed elsewhere, the Fireman had to check the train on left hand curves.
5. A fireman had a lot of things to watch, other than the fire -- water glass, steam gauge, etc. and had to keep the boiler water at the correct level for the terrain, periodically blow down the boiler, and, on oil-fired steam engines, sand the flues at appropriate intervals in appropriate locations. On some railroads' engines, he also controlled the headlight and/or had the manual bell ringing duties. All those valve handles on the left side of the backhead were his responsibility.
6. None of his observations were made through glass, normally. They were made while leaning out of an open cab window, or while standing in the gangway.
7. About half the time, this was all done with little or no cooperation from the Engineer. In the steam era, a lot of Engineers were unfriendly, cross, and critical toward Firemen. A large number of Conductors treated their Brakemen the same. Some Engineers and Conductors were fine gentlemen, but a large number had a somewhat regal view of their status on the crew.