Good to read that you enjoy the LME49720 so much!
The only way to actually measure the DC voltage is by using a multi-meter. The ones you can buy at sub $10 price levels are usually not the most accurate ones, but accurate enough for this measurement. Something like this:
http://www2.conrad.nl/goto.php?artikel=122999 would be perfectly ok.You can often buy them at Home DIY shops and car equipment shops.
Another way you could test this is by inserting the headphone plug into the D1 with an unused input selected. If there is DC on the output you will hear a noise when you 'click' into the first contact. The sound can vary from a clear click (=high DC, potentially dangerous for your headphone) to a vague noisy plop (=low DC, probably harmless). This approach is not very scientific, but it might put your mind at ease (or not
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