There is no log message for guarding an attack; the only reliable way to tell is the absence of blood after an attack on your character.
Guard only works when the player is using a Hand-to-Hand weapon (or barehand).
The higher your Guarding Skill, the better the chance that it will activate. Adding AGI was thought to increase its activation rate, but has not been shown definitively to do so.
Guard Activation and Effect
Opponent's attack must bypass the character's evasion, shadows (Blink/Utsusemi), and counter for this trait to trigger.
Guard reduces damage by reducing the pDIF of the attack by 1.0.
This trait can activate on enemy TP attacks, completely nullifying the damage. The log will say "(Monster) uses (TP move), but misses (Player)". It is also possible to get a guard skillup from an AoE TP move targeted at someone else in the party (as long as Hand-to-Hand weapons/bare hands are drawn and facing the enemy).
A guard proc on multi-hit TP moves will NOT nullify the full damage; it nullifies only 1 hit of a multi-hit TP move (though it is possible to proc on more than one hit).