Cepstral Community

How can I find out what phonemes are being used to render text into sound? For example, I can make Alejandra speak a credible Mexican accented English word "I" by typing "Ae" (hey, I knew latin would come in handy some day). Now if I were to want to put that into the lexicon file, so I can get her to do that without tricky spelling, I'd need to know what phoneme(s?) "Ae" is generating. So is there a way to do that?

I tried having her speak all the words in the examples in the lex file but I didn't hear any long i sounds.

Just to be absolutely clear, while the specific issue is of real interest, I am looking for a general solution if possible, not just for the phoneme for this one word.