I agree with Kristin. Since you really should not touch them, why would it matter? If it is a matter of moving the toads, coax them into a small container. I sometimes let them jump onto my hand for a short period. That way, only their feet are touching me. In my case, however, I know my M. stelzneri are no longer toxic, so my concern is with harming them, not me.
As to the specific question, I am not sure I have seen an answer. The only way to definitively know is to determine precisely when they were captured, what food they had on the journey, and then chemically test the toxicity of the toads in a scientific manner at regular intervals. You would also have to define what is "safe" toxicity since all frogs and toads have some toxicity.
It sounds like an interesting project. Any volunteers?
Chris D.