if no battle has been made this is the only way to find out fortification...but melee and armor are impossible to find out, just put more than you think they have to the calc. to be sure...
Melee is easy if he/she fortress siege attacks you or one of your allies. Check the damage done from soldiers. You can look at the battle history and see what he/she had 2 weeks ago. For gold mines the battle history is usually correct.
Once you have the mellee you can run battle calc a few times to estimate the armor.
It is best to assume that they have added some research recently. I also assume that he/she will log in and add a few thousand troops from the barracks and move archers to the fort.
I try to find battles where I annihilate the enemy. If it takes 3 rounds to smash the fort instead of 2 I will survive.