Can a EtG test tell if it's not my urine? Can they tell if it's synthetic urine

The answer is a little more complicated than it may seem. At the point of collection urine should be checked for temperature and coloration. Sometimes the collection cup can detect a range of temperature, or a thermometer can be utilized. Many times it is simply experience of the technician. Temperature is only a factor within minutes of production.

Once the sample is obtained it could be tested by immunoassay (IA) a dip-stick type test conducted at point of collection. This system uses an embedded antibody that responds, in this case, to ethylglucuronide (EtG). If the IA returns a positive then the sample is forwarded for more extensive confirmatory testing. If the result is negative by IA the test could and usually does end there. IA testing is generally not sensitive to elements of urine content beyond the antibody and, sometime, creatinine. IAs have a high error rate.

However, the second part of the test for initially positive IA samples is conducted by highly efficient instruments at a laboratory, Gas Chromatography-Mass Spectrometry (GC/MS). This system analyzes the most minute particles of sample content. It can measure known variances of human urine: creatinine and specific gravity in particular. In broad terms this assesses the density of urine and is used to determine if the sample has been diluted or alien. But the instrument can also detect virtually any type of molecule. So if synthetic urine contains one or more molecucles that distinguishes it from natural urine that can be picked up in the test. (Depending on the test, IAs, can detect creatinine).

The answer to your question is Yes, they can. Whether most do is another question. Much is dependent on the quality of the IA (the initial test), the settings of the GC/MS and the level of competence of testing personnel throughout the process.

Urine substitution and use of synthetics is not uncommon and testing agencies are well aware of that. But the testing field has expanded so rapidly that the quality of products and services has suffered immensely. Laboratories and systems range from incompetent to highly proficient.

You must also be concerned about the quality of synthetic urine. Dependent on the manufacturer it could be easily detectable by GC/MS. That field has expanded at a rate equal to the testing field. I'm sure product quality varies to the same extent as testing, although I have no direct experience in evaluating the products used to compromise the tests.