Dan
Without knowing the full topology it's difficult to say for sure but as a general answer you can't really.
Even if you tried to set the AD of OSPF to be the same as the static route, which I'm not sure would be a good idea, the locally configured static would have a better metric.
I think you are basically going to have to test by advertising out the default route via OSPF, making sure it is in the OSPF database at each site and then remove the locally configured default to test internet connectivity via your main branch.
Or you could try using PBR to bypass the routing table which would allow you to test central internet connectivity but it doesn't confirm your OSPF routing is working properly.
Jon