When I go to a service change, I look for the GEC if it looks like there is a CEE but there is no access to it ( blank cover) I pull my service out look down the cavity and 9 times out of ten I can see the rebar and ground clamp I cut me a hole were the ground clamp is , put a 2 gang low voltage cut out box with a bell box cover. Now it is accessible. That's what I did today but instead of rebar going into the concrete it was bare # 4, if you take off blank cover you can clearly see it going into the concrete. I ended up getting it passed, I guess they are used to seeing ground rods, I find for me that it is easier to drive one ground rod right under the panel, then to try to locate the existing CEE, but most AHJ want you to drive 2 that's when it becomes a headache for me, I do whatever is easiest at the time