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I would guess your WAN connection hand-off is Ethernet, if so, the connected interface show interface stats should show the physical connection speed.
WAN Ethernet often has logical bandwidth caps applied by the WAN vendor. They might be "soft", i.e. you can exceed them (often with a surcharge if done) or with "hard", you won't be able to exceed them. The latter can be confirmed by using traffic generators. Push gig amount of traffic into the near side and "see" how much comes out the far side.
