It is not possible to determine if a crop is transgenic or not just by observing with the eye or by any other easy way. However, it is possible to determine by analytical methods whether a plant contains foreign genes or not, up to a limit of 0.1 %. Only a small sample is needed to obtain an accurate determination.

First a qualitative analysis can take place to see if the material contains any genetically modified genes. Normally the product is analysed for typical sequences, which are used in the production of transgenic plants. An example is the 35 S promoter, a sequence that is added for the regulation of the expression of the inserted gene. Another element that is often looked for is the NOS-terminator, another frequently used sequence in transgenic plants. In that way it can be determined if a crop is transgenic or not. A quantitative analysis can then take place to determine exactly in which way the sample is genetically modified.

Nowadays there are several methods available to determine foreign DNA in samples.