It depends a lot on the nature of the VPN you're asking about. If you're talking about something like TOR, there are well-documented (PDF warning) ways to check the user's IP against exit node lists, and if we're talking about a corporate VPN or specific VPN services you can identify exit IPs for the same would apply, but I'm not sure if there's any way to reliably tell if I've VPN'd into my home network from work or any similar setup.
That looks like its a pretty good security site for checking yourself out. As far as how easy, well it's extremely easy for companies. Just block all commercial IPs.
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