Take a look at the left (as seen from behind the steering wheel) side of the block. Just under the head should be a cast-in number starting with 4969 followed by two more stamped in numbers on elevated surfaces. The first number, e.g. 496900, tells you what type of engine it is and for which model it was intended. The second stamped-in number is the serial number.
You might have to wipe dirt away to be able to read them.

From left to right: engine no.; engine-type; max. SAE-bhp; type of camshaft; model in which it was installed.

496900 B 20 A 90 A Amazon
496901 B 20 A 90 A Amazon
496910 B 20 B 118 C Amazon
496911 B 20 B 118 C Amazon
496922 B 20 B 118 C P 1800
496923 B 20 B 118 C P 1800
496924 B 20 B 118 C Amazon
496940 B 20 E 130 D P 1800
496941 B 20 E 130 D P 1800
496942 B 20 E 130 D P 1800
496947 B 20 E 130 D P 1800
496949 B 20 E 130 D LM
496979 B 20 A 90 A Amazon
496980 B 20 B 118 C Amazon
496982 B 20 B 90 A Amazon
496995 B 20 A 90 A LM
496996 B 20 B 118 C LM
496997 B 20 D 105 B LM

Source: www.amazoncars.nl. This list is not complete, strangely enough this site did list several B18s intended for the 140-series, but did not do the same for the B20.
LM means that the engine was supplied to a manufacturer other than Volvo.