How can you roughly tell which region of the world a global unicast IPv6 address comes from?

Global unicast lives under 2000::/3; within 2001::, the next digits map to a Regional Internet Registry — e.g. RIPE (Europe), ARIN (North America), APNIC (Asia-Pacific).

* Structure of a global unicast IPv6 address. — Michel Bakni, CC BY-SA 4.0, via Wikimedia Commons. *

All normal, internet-routable addresses fall under 2000::/3 (i.e. they start with 2 or 3). The early 2001:: allocations were handed to the regional registries in recognisable blocks:

This is only a rough heuristic for older blocks — modern allocations are more mixed — but it shows how the registry hierarchy is reflected directly in the address bits.

Tip: The five RIRs worldwide are RIPE (Europe/Middle East), ARIN (North America), APNIC (Asia-Pacific), LACNIC (Latin America), and AFRINIC (Africa).