After having lived in Bangkok for three years and annually extending my Non Imm O visa for retirement (pension) during this time, I have left Thailand for my  home country in the US. I won't return until after my extension expires in January. I plan to use the visa exempt upon entering Thailand and then apply for the Non-imm O visa again to essentially start over. Question:   Will I need to verify my pension with the US Embassy in order to obtain the Non-Imm visa? I know I will need it when applying for the extension after the 90 days is complete. 

You have to prove your income to apply for the 90 day non immigrant visa entry at immigration. You will need at least 15 days remaining to apply for the visa. Also not all immigration offices can accept the application.

It would be best if you got a single entry non-o visa based upon retirement at one of the honorary consulates in the states before returning. You have to apply in person at most of them. There are reports of the consulate in Portland accepting mail in applications. List of them is here: http://thaiembdc.org/royal-thai-honorary-consulates-general-in-the-u-s/