As we speak, the CDC is testing the substance found in the Moore-Hill dorm at UT for ricin. I've been trying to figure out how exactly they confirm the presence of ricin, and luckily, the CDC (sort of) publishes its methods.

According to this page at the CDC, they use two methods for confirming the presence of ricin in environmental samples: PCR, and time-resolved fluorescence immunoassay (TRFIA).

PCR, of course, is a tool that most molecular biologists use all the time. It's a simple way of specifically amplifying a targeted DNA sequence. For diagnostic purposes, the CDC is counting on residual castor bean DNA being left in the ricin. They attempt to amplify it and, if they get amplification, they can confirm that the substance contains the gene(s) that produce ricin. This is probably not the most conclusive or sensitive test, because highly purified ricin should contain little DNA, and because the test doesn't confirm the toxic substance, it confirms the DNA that makes it. (It's also possible to get false positives, but this should be a relatively rare event because of the specificity of the PCR primers.)

An ideal test would not just confirm or deny the presence of castor bean DNA, it would quantitatively determine the number of toxic ricin molecules directly. And this is where TRFIA comes in. For those familiar with immunoassays, TRFIA is a very sensitive, fluorescent analog of ELISA. With TRFIA, one immobilizes the target compound and binds it with a primary antibody; after washing, a secondary antibody-Eu³⁺ conjugate then binds the primary antibody. The europium (Eu³⁺) has an exceptionally long fluorescence lifetime. To make use of this, the sample is pulsed with excitation light, then after a long delay (~400µs), the emitted fluorescence is measured. This is performed repeatedly, and only the compounds with very long fluorescence lifetimes on the average give fluorescence. This removes essentially all background fluorescence except that of the Eu³⁺. The result is a very sensitive immunoassay for confirming the presence of any antigenic substance.

The sensitivity, of course, depends in part on antibody affinity for the target compound (e.g. ricin), but the technique has demonstrated linearity over ranges of pg/mL to tens of ng/mL. It's also interesting to note that the CDC must keep stocks of anti-ricin antibodies. Presumably these aren't polyclonals obtained from animal exposure to ricin. Keeping that cell culture alive sounds like an important job!