I've read and heard that they are unused but what you are getting is not new in terms of production date. It's old stock because Panasonic produced them in bulk batches and once they stopped, that was it. From what I understand the packs get charged and then shipped with a fast option so they can get installed to be used immediately. The stock could be from 2006 when the Insight and 1st Gen Civic production stopped, but that is only speculation on my part, could be sooner than that. The new sticks from Honda starting with the 2nd Gen Civic hybrid are designed without PTCs and in an incompatible double-stick form factor that the 1st Gen Civic and Insight cannot use. Panasonic requires a huge quantity to begin production and I don't see there being any chance of new packs being produced.
You asked about what you can check. The definitive answer comes from the check engine code. Find out the code and post it here or do a quick search and if its a common IMA code, you'll find it either here or from Google with Honda Insight Pxxxx with the code replacing the P code you get. One important detail, the Civic Hybrid will likely have a 12v battery light triggered when the IMA light is on, you might not get too far before the 12 volt battery discharges and you can't drive it any farther.
FWIW, $1505 sounds like a much better deal than most Insight owners are getting for an out of warranty replacement.
Since you are on the Insight forum, we'll prod you to get an Insight. Do you require the back seat? If not, you should get the 1st Generation Insight, gas mileage is nicer on this side of the world.