How do I check if seed in a bag has been mixed or is impure?

You can do a simple purity check by hand even though the lab does it precisely. Spread a handful of seed on a clean white sheet and look closely. Good pure seed of one variety should be fairly uniform in size, shape and colour. Watch for clearly different-looking seeds (another crop or another variety), weed seeds, broken or shrivelled seeds, stones, soil clods and chaff. A lot of foreign matter or many odd seeds is a bad sign. The certification tag also lists a minimum purity and germination standard the lot is supposed to meet. If you see heavy contamination, do not sow it, keep the bag, tag and bill, and show it to the seed inspector or your KVK. For a precise purity and germination report you can submit a sample to a seed testing laboratory. Confirm the purity standard for your crop with your KVK if you are unsure.