You don't. You hire competent legal counsel barred in the State you live in. He or she will listen to your situation and adopt an appropriate pre-trial and trial strategy.
Note that even those mental health workers that consider mythomania a real condition admit that prevalence is quite low (one researcher shows only 1 case in 1000 in incarcerated juvenile offenders- a very high risk group- this would mean the general population would have far fewer), that the diagnosis is extremely controversial at best (meaning that few in the mental health field consider it a "condition" worthy of study or research) and the nature of mythomania is to lie at all times and with no discernible motive, not simply when it profits the sufferer to do so.
Unless your spouse has a diagnosis of mythomania from a competent clinician, any judge is likely to consider your insistence that he or she is so as an act of histrionics on your part.
It's very difficult. Only correspond with the liar through e-mail so that you have everything in writing and proof if you need to show it. Do not have phone conversations with the liar. I made that mistake. Only correspond in writing or through attorneys, period.
Am going through this at the moment. Best way to expose is to obtain every single thread of evidence to disprove statements being made. Thia can be by way of credit card and bank statements, reciepts of purchases, calendar entries, even looking at when documents were created on a PC will give you an indication of events at that time. Also don't forget photographs, home videos, videohire listings, utilitiy and phone bills, right down to Dr's & prescriptions etc.
Also, I have a ton of friends all writing letters detailing their own experiences with the woman and their perceptions. This gives courts a "helicopter view" of the whole situation.
Does it work? We'll my 6 yr old son and I sincerely hope it does in the end.
Get a voice recorder, they cant deny the voice they own & people are most likely to really "go off" in person or on the phone. But check your states laws on recording. I have one I use every time I speak with my ex, it's small enough to fit in my purse or pocket, cheap to but worth millions.
(06/13/09) The use of a recorder depends on state law, but also you need to transcribe the tape. A recording can be prevented from entrance as evidence, until they commit perjury, but you can enter the transcription. You read it to them while they are on the stand and ask them to confirm it. If they deny it, than you hold up the tape. You may not even need to enter it into evidence as they their reaction may be enough.
If it is legal to tape in your state, always use new tapes, never tapes used previously. Be sure the state date, time, and conditions under which the recording was made, at the end.