High-quality sweet potato vermicelli should be made from 100% pure sweet potato starch without the addition of corn or cassava starch. You can identify purity by the color, which should be translucent greyish-brown or slightly yellowish, rather than bright white. When dry, the noodles should be brittle and easy to break; if they are too flexible, they may contain excessive chemical additives or alum. Upon cooking, the texture must be chewy, smooth, and resistant to overcooking, maintaining its structural integrity even after being boiled for 10-15 minutes.