Depending on how long it's been, a lot of things are still likely the same. Map maker URLs and GMB URLs can still get you where you're going with the old formats, the most important thing to find is the CID. There's a couple of ways to do that.
You can go to map maker and look up the business there. Right click the business' name, copy address, and the fid is in the URL. You can use this tool to convert the fid in the URL to the CID you'll need, the tool will even spit out the URLs to the G+ page and so on.
If you pull up the business in maps (either by searching for the business by name, or by clicking on the map in the knowledge panel) you can view source and ctr+f for the cid. If you find the plus.google.com profile, you can view source there as well.
The biggest change you might not be aware of, unverified listings no longer have a plus.google.com page (they're no longer indexed, and even if you find it, it 301's you straight to maps.google.com), so the sitelus.google.com + phone number trick for finding duplicates doesn't work anymore. You'll want to use this as your current best practices for trying to find duplicates now instead (the main new step being that maps.google.com has started being indexed for unverified listings).
As far as how to tell if a profile is a real business profile or something else, one of the easiest quick tests is to look for the CID. No CID in the source, then it's just a normal plus page, or a ghost from a past merge or something. Hope that helps you get started at least.
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