The inconsistency of an opponent’s position can often be demonstrated by substitution. Replace the groups, persons or phenomena which are the "actors" involved in the proposition they are arguing for (or against), with other groups, persons or phenomena for which they hold a different (often inverse) position. Assuming that the chain of reasoning for the argument ought to be comparable, it follows that a comparable conclusion ought to be reached.
For example, Lee N Wright is arguing with his girlfriend Ima Green about the justification for military pre-emptive strikes on terror cells and rouge states:
"We might not be 100% sure that there will be a terrorist attack, but the precautionary principle means we ought to act before it’s too late."
Ima is aware that Lee is an anthropogenic climate change skeptic who thinks there is no need to worry about greenhouse gas emissions. She points out Lee’s inconsistency by simple substitution:
"We might not be 100% sure that climate change is man-made, but the precautionary principle means we ought to act before it’s too late."
If Lee argues the precautionary principle in one case, to be consistent he needs to uphold it in all cases. He either needs to back away from his argument for pre-emptive strikes, or alter his position on climate change. (This assumes there are no significant differences in both cases, otherwise one could be guilty of a False Analogy.)
Note that Ima is in the exact same position as Lee, given she thinks we ought to act to minimise man-made climate change - I often wonder how people are able to function whilst suffering from obvious cognitive dissonance.
Examples:
The Skeptic's Field Guide by Theo Clark and Jef Clark is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.