In the old days.....
OS map, ruler, large sheet of paper and sliderule.
Not so long ago....
OS map, MS Excel.
Very recently....
OS map, Only look at the hills with a chevron.
Now…..
Streetmap.co.uk Only investigate what I see as steep ones.
I have a Sony 15" laptop. I am so lucky that Streetmap at 1:25,000 gives 500m on the map = 5cm on the plastic ruler. If your screen does not match up with a plastic rule, make a dedicated scaler with a piece of cardboard. I had to do this with a 4:3 ratio Dell monitor.
I scale off the distances between the contours and construct a little Excel table of Dist vs Elevation.
Contours 5mm apart on the screen = 10%. When I see a bunch of contours less than 5mm apart, I investigate it.
This is about as accurate as I've been able to get.
Measuring the total climbing of a 200 km Rando is a balls aching task. A 100 km Populaire is a lengthy process, so I don't bother. Only 10% and steeper so I can prepare.