Depending on the project specification and ground conditions, the engineer may be content with drillers descriptions and SPTs for the borehole logs. Therefore, the descriptions will not be checked against laboratory tests. However, if subsequent lab tests were to indicate a soil different from that visually described, BS5930 Code of Practice for Ground Investigations recommends that the description should not be discarded on that account but should be preserved as a record of the observers opinion.
So how can we be certain of the accuracy of field descriptions without lab tests? Well that will depend on the training and experience of the driller with in-house lab tests completed from time to time to ensure the accuracy of the logs. Annex A of ISO14688-1 Identification and Classification of Soil acknowledges this where it encourages persons to compare their descriptions to laboratory tests to check the accuracy of their fieldwork.
Anyhow, this question was posed of late between team members when apparent horse manure was found during sampling and how we knew that the sample was in fact horse manure and how were we going to record it in the drillers logs. Funny enough, the drillers assistant, a farmer, demonstrated the fact through the hairs and barley oats which had passed through the animal; barley oats being the staple diet of the working horse. The site was originally an old bakery in the early part of the 20th century and the bread was distributed across the city via horse and cart. One sample picked up the interface of the made ground and boulder clay perfectly.