Pregnancy tests for viruses

There are several ways to check if your plant is infected with Tobacco mosaic virus (TMV). If your plant of interest is Nicotiana benthamiana (i.e. such a sickly thing that it wilts at the mere sight of a virus) then you wait for them to die. Though that could mean it was infected with any one of scores of plant viruses.

To narrow down the list of culprits, and to do so before plant death, then look out for the dark green veins surrounding light green patches of leaf tissue, where chlorosis has occured due to insertion of the virus coat protein into the chloroplastic envelope. The leaves will become mottled and curl up. These symptoms could be attributed to a much smaller number (i.e. less than 10) of different virus species that cause the same symptoms.

If you're more pedantic about your viruses and want to know if your plant has TMV, you could use antibodies against TMV-specific proteins, or primer sequences that complement TMV-specific RNA sequences in molecular-based assays.

However, you can also get tests to check if your virus is TMV/CMV/boy/girl/indeterminate gender. They look like a pregnancy test, work in the same way as a pregnancy test, you interpret your results in (I'm told) the same way as you would a pregnancy test. The only difference is that instead of peeing on a stick you put plant extract on it. (And no, you cannot train plants to pee).
And yes, my brain is addled from too much data analysis.