tCheck only measures delta-9 THC and cannot measure delta-8
When people mention THC, they are referring to delta-9 THC. At the molecular level, delta 8 and delta 9 are extremely similar - just one of the carbon double bonds has been moved. To make delta-9 (simply called THC), just take THCa that is naturally produced by the flower and heat it or decarboxylate it. To make delta-8, you have to go through a process called isomerization which involves acids, reagents, and solvents.
Delta-8 gained in popularity in 2020 following the 2018 Farm Bill that legalized hemp. After extracting the CBD from hemp, the by-products from extraction is then transformed or isomerized into delta-8.