Worked example - decoding the K+ meson. You are told a particle has electric charge +1, strangeness +1, and is a meson (so it is one quark plus one antiquark). Step 1: strangeness +1 means it contains an anti-strange quark s-bar (a strange quark s would give strangeness -1). Step 2: the s-bar has charge +1/3, and the total must be +1, so the quark partner must carry +2/3 - that is an up quark u. So K+ = (u s-bar). Check: charge (+2/3) + (+1/3) = +1, strangeness: s-bar contributes +1. This is exactly how physicists used conservation laws to work out quark contents in the 1960s.