To identify the bore sizes you first have to understand what bore sizes were available in certain years.
In all cases the bore sizes is either stamped on the valve piston casing, or there is no mention. Horns without a bore size a re usually Medium Bore instruments.
Martin Handcraft (Pre Standards/Imperials in 1934)
S = Small
M = Medium
L = Large (Yes, large bore models existed even pre 1920's)
The marking is typically on the opposite side to the serial number and will be stamped on the middle valve. Look for the S, M or L
The above image demonstrates the most common variation for a valve block and the positioning of the bore size. (This is a Martin Handcraft Imperial earlier model in case you were wondering)
Martin Handcraft Imperial/Standards
1 = Small .445
2 = Medium .453
3 = Large .468
4 = Extra Large .480 (Not commercially available - but instruments exist. I own one)
Martin Handcraft Committees and Committees
1 = Small .445
2 = Medium .451 to .453 Bore
3 = Large .468
Martin Imperials & Indiana's
Do not have markings - but were available in a Medium .453 Bore
Later model RMC Indiana trumpets were available in different bore sizes.
Martin Custom Committees
These are .460 if no markings, or Large Bore .468
After 1960 and serial 700000 - you typically find the serial and the bore size on the side of the valves where your thumb or index finger might sit. ROtate the horn and stare down the leadpipe or bell and you should see the markings.