Look on the top of the cylinder head at the back, just aft of the valve cover flange. Here you may find a large raised number in the casting. "15" was used for the 1500 and 1600 engines. This one could be a bit of a fooler though, because it was also used for the 1500 engine in small valve single carburetor cars like the Nash Metropolitan, and with that you might get much smaller intake valves and the performance of a 40 hp VW Beetle. Bummer, but it's hard to tell without removing the cylinder head for inspection. There are certain casting numbers on top of the head inside of the valve cover, but lots of variations there, and no telling if a rework has changed the valves to a larger size in a past life. "16" was used only for the 1622 engine of the MGA 1600-MK-II.

"18" was used for the MGB 1800 engines through 1971. Starting in 1968 all MGB heads (at least in the USA) had air injection ports near the spark plugs. "L" was used for MGB heads 1972-1974, and those had larger intake valves and higher compression (but single valve springs rather than dual). Then back to '18" for later heads. Some replacement heads had no number here, and you have no idea what valves those might carry. The 1975 and later heads had a water take-off port on top at the rear corner to supply the hot water controlled choke on the single Z-S carburetor of the rubber bumper MGB.

For more detailed information on specific cylinder head casting numbers, see CH-100 in the engine section.