How can you tell if a Ardor / NXT public node is an archival node? I'm looking to build a chrome app that queries the ardor / nxt decentralized data cloud using peerexplorers api public ip addresses here: http://www.peerexplorer.com/peerexplorer-api

The question I have is how do I tell if an IP address is an archival node? What can one query to find this out? 

Let's say I wanted to aggregate all the torrent files that were ever uploaded to the ardor / nxt data cloud. How can I find archival nodes to query this?
There are a few ways to do this, but I'll describe the most straight forward one. Find an archival node from http://www.peerexplorer.com/api_openapi_archival.

If you pick any one of the ip addresses in that list you can then query that node to verify if it is a archival node by calling its getBlockchainStatus function, for example: http://23.94.134.161:7876/nxt?requestType=getBlockchainStatus

In the reply you will se a list of services, where 'PRUNABLE' is the one you are after. Any node that has PRUNABLE in their services list is an archival node.

On a side note, you don't need to query a archival node in order to get the actual data. Any node with open API will do, as long as you specify the retrieve=true parameter which is added to most relevant API functions. By doing that on a non archival node, it will locate the data from an archival node and return it in the query. With a bit higher latency of course, since there are multiple network calls that needs to be done.