Another way to tell is the color (usually shows in the name). Different strains of kratom have different colored veins on the leaf. The colors can be Red (Slow), Green (Moderate), or White (Fast). So a strain like ‘Green Kalimantan’ is a moderate strain from Kalimantan (Indonesia). The location is still the best indicator though, as odds are a 'Red Thai' strain would be Fast.
Some vendors actually organize the speed of their kratom on their site, which can help if you're unsure.