It might be they'll know when they received the shipment that's on the shelf. Whatever arrived after March 26 would be the new version. It could be they are aware of it, or could make a phone call and find out.
Ask to try it out before purchase and run through the tone modes, looking for the dP mode that comes up after 4-tone mode.
Buy it anyway, as the revision isn't critical or absolutely necessary and you have a 50-50 chance of getting the new one. If it's the old version, try it out and see if you'd like the added features, then return it for the upgrade. It'll cost $49, but since you're getting a good price, you may still come out ahead.
Ask how quickly stock is being sold and reordered, "how popular is this thing, anyway" sort of question. The new revision has only been out less than a month, perhaps waiting a few weeks would help assure it's new stock.
Talk a buddy into buying one and find out from him which one he got! But, that would be cruel!
Lastly, we haven't heard yet if new units are being shipped in updated display boxes.
-Ed