If you look at the particle data group tables, mesons for example you will see for each resonance listed its parity, in the same line as the name and the mass of the resonance. Like the mass, it is an observation from measuring the resonance in experiments, laboriously in experiments over the years .
for example, the parity of the pi- was measured in a specific experiment:
The reaction π−+d→2n has been observed by detecting the two neutrons in coincidence with slow negative mesons incident on a liquid deuterium target. The observed angular correlation of the two neutrons confirms the identification of the process. The process is therefore not forbidden, and this fact may be used to establish the odd relative parity of the pion and the nucleon.
Received 8 June 1954
So it is angular correlations in combination with known parities that establish the unknown ones.Once there is a list of known parity particles and resonances, the parity of new states can be measured , as is proposed to measure the parity of the Higgs in future experiments.