golang - how to access internals of struct (reflection?)

I need to add attributes to a log15 logger, but if I inadvertently add one twice it will appear twice. So, I want to add logic to see if the attribute is already populated and skip the action if it is.

Here's what I get when I output the log object:

log="&{ctx:[field1 val1 field2 val2 field3 val3 field2 val2] h:0xc82052c1e0}"

How can I access that 'ctx' field and validate when my value is already there? I've tried some reflection tricks and can get the data type, but I can't figure out how to get the value.