The closest thing to a "secret trick" on guitar IMHO is to learn (1) the diatonic intervals(!), and (2) the fretboard, i.e. how these intervals happen to interact with the guitar tuning you're currently using. Start by singing ut, re, mi, fa, sol, la (solmization) and find the notes on the fretboard as you do so.

(You may notice that we don't use ti or altered solfège syllables: that's because it's convenient to keep mi-fa as the only marker for a half-step and use an exceptional hexachord mutation whenever we need to reach other notes. (For example, the full major scale is sung ut, re, mi, fa, sol, re, mi, fa and descends fa, mi, re, sol, fa, mi, re, ut. Note how the half-steps are consistently mi-fa and fa-mi. Centering the system on that one feature agrees with the guitar's nature as a relative instrument; unlike on the keyboard, we need not think by reference to a single diatonic scale and its 'sharp' and 'flat' notes.)

The system also extends cleanly to other intervals; for example, the minor third is just re-fa or mi-sol, the major third is ut-mi or fa-la, etc. Very easy.