For a conventional model the things I use to determine how exact the CG point I am using is is to do a few stalls, (at height). If it drops a wing sharply or becomes extremely unstable or uncontrollable as I get close to stall the plane is normally tail heavy. If it just drops its nose and continues then it is nose heavy! The other thing I look at is the elevator setting for level flight. If the elevator is pointing downwards when in the centre position the plane is tail heavy and if it is pointed slightly upwards it is nose heavy. Mind you the aircraft weight and wing incidence angles can also cause a plane to trim for level flight in a nose up or down attitude.
A canard is best judged for correct CG in a full up elevator/elevon, power off glide. If the plane stalls it is too tail heavy and if it drops its nose and seems to porpoise it is too nose heavy!
A wing is also checked in a full up elevator/elevon glide. If it wanders from side to side and even drops a wing at stall it is too tail heavy whereas if it lifts its nose and then drops in nose again in a rapid series of repetitive motions the wing is actually a little nose heavy!
Just what works for me!
Have fun!