David:
If your locomotive has drain cocks on the cylinders you can time the
valves by slipping a piece of tubing over a dran cock with the free end
in a container of water. Put a bit of air pressure in the boiler and
turn the wheels and you can see bubbles when the valve opens which
should be right at dead center or juat a bit before.

The other way is visual and done by taking off the steam chest covers
and looking at the valve while turning the drivers. When the wheels
reach dead center the valve should be just cracking open. (As LBSC
said, "Open the thickness of a tram ticket" (Being in the US I have no
diea of what thickness a tram ticket might be, but assume a thin
cardboard.)