When sourcing mushroom spawn, the most critical factor is the substrate type (grain, sawdust, or plug) which must match your final growing medium. You must verify the strain purity to ensure high yields and resistance to pathogens. High-quality spawn should have a vigorous, brilliant white mycelium growth without any signs of yellowing or dark spots, which indicate aging or contamination. Additionally, check the generation number; lower generation spawn (G1 or G2) typically offers better genetic stability and productivity than higher generations.