Freeze my own urine or use synthetic?

[Burnt Toast]

Are there any tests that they can legally use, even if only on very rare occasions, that will detect synthetic urine, or will at least detect the absence of something in the synthetic urine, and thus cause a person to fail the test?

No there is not. Under DHHS regulations, labs determine a "non-human" sample by checking the creatinine and specific gravity. Under DHHS criteria, a "non-human" sample is ruled when the creatinine concentration is less than 2 mg/dL and the specific gravity is less than or equal to 1.0010 or greater than or equal to 1.0200 on both the initial and confirmatory creatinine tests (i.e., the same colorimetric test may be used to test both aliquots) and on both the initial and confirmatory specific gravity tests (i.e., a refractometer is used to test both aliquots) on two separate aliquots.

As long as the synthetic batch is unexpired and had been stored properly (ie no exposure to direct sunlight), it will pass the validity checks permitted by DHHS.