The main way is the country or province the strain is from. Bali and Borneo are slow strains, Thai strains or ones called 'Maeng Da' are fast, and Indonesian strains are generally moderate.
Another indication is the color of the vein of the leaf of that strain (usually shows in the name). The colors can be Red (Slow), Green (Moderate), or White (Fast). So a strain like ‘Green Kalimantan’ is a moderate strain from Kalimantan (Indonesia). This indicator isn't always correct, but in most cases it is. If you’re confused, at the end I show exactly which strains are what speed.