A boiler replacement is easier to judge when the house, the pipes, and the controls are on paper before anyone picks a unit. Take the questions below in this order, and do not let a quote answer them for you.
The heat load on paper
Ask for a written heat loss calculation for the house, room by room or for the whole structure. The size of the old boiler tells you what the old system was built to deliver. It does not tell you what the house needs now. If the insulation, the windows, or the added rooms have changed since the old unit went in, the old rating is a poor guide.
Ask the installer to show the calculation and to explain how it was done. If no calculation exists, write that down as an open item.
The piping and the radiators
Look at the radiators, the baseboard, or the floor loops the boiler will feed. Check the pipe sizes where you can see them. Ask whether the system has ever been flushed, and whether the water that comes off a drain valve shows sludge or rust. Ask whether any part of the distribution must change, and whether that change appears in the written scope.
The controls you keep
Note the thermostat type and its wiring. Note whether there are zone valves, circulators, or an outdoor sensor. Ask which controls the new boiler will use, which ones stay, and whether any wiring must be added. Get the answer in writing, because controls are the part of the job most often left out of a short quote.
Venting, expansion, and removal
Ask who sizes the venting and the combustion air, and whether that work is inside the scope or listed as a separate change. Ask about the expansion tank, the relief valve, and the drain for it. Ask how the old boiler is removed, who disposes of it, and what the site looks like afterward.
Paperwork before the order
Ask who pulls the permit and who schedules the inspection. Ask for the written scope, with what is included and what is not. Ask what the manufacturer's warranty covers and what the installer's own commitment covers, and keep those two in separate notes. The boiler page turns an install year into a planning window, which helps you decide whether the existing system is worth tying into at all.