Full Idea
Kripke offers an intuitive test for telling whether a term is rigid: try the term in the sentence-frame "N might not have been N". (For example, try the terms 'Nixon' and 'President of the USA').
Gist of Idea
Test for rigidity by inserting into the sentence 'N might not have been N'
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by William Lycan - Philosophy of Language Ch.4
Book Reference
Lycan,William G.: 'Philosophy of Language' [Routledge 2000], p.54
A Reaction
Helpful, but if you try it, the results do not seem to be conclusive. You are left saying 'Well, it depends what you mean by...' Think of possible worlds with a crippled Nixon, twin Nixons, an honest Nixon, a robot Nixon, a dark skinned Nixon...