Use the F10 screen, unless you have poor relations (<-25 if I recall) that should show the relative military power of the main states. Under the 'power' column, first number is land and second is naval.
You can also click on the state (first line of F10) and get more detail of how their overall score is made up.
I'd guess that powers to watch are Britain, France, Austria and Russia.
Having said all that, double check the requirement. its a, not really explained, moving average over the opening 10 years.
In other words while I know how to check relative military strengths. prestige progress etc i've never really worked out how to track progress for the at-start goals ... and never managed to achieve one either
