How to test used / ex-demo speakers?

You could try a test CD with test tones and frequency sweeps to get the drivers and tweeters activated in a short period of time. Plus you can control the tracks listened to and how often and at what sound level, repeat etc. Most music is less dynamic than the sound test cd's which are designed to stress a sound sytem and break it in and use full frequency tones. Take you favorite music, fast, slow, loud, good recording and bad recordings too. Try to get at least 30-60 day warranty if not better. Good Luck!

Listen for imaging and soundstage.

What speakers are they? Is this a model that has been around for five years? Do they provide customer support?

If so then you can always repair/maintain them. Not many manufacturers make their own drivers and are consistently able to offer good support ten or many more years down the road - but the well known names do - just avoid a "boutique"name if you are worried about long term reliability/service.

I'd prefer to buy used/demo any time. So many speakers need several hundred hours to break in. Let the dealer do it for you and still give a warranty!