One of the ways that musicians make accented beats stronger than the normal beats is with pressure. Percussion instruments require a heavier strike on the drum or key. Horn instruments need pressure in the form of stronger breaths into the instrument's mouthpiece. String players strike or pluck the strings harder. The goal is to make a sound that is audibly more present than the other notes played in the piece.
The accented beats are also louder than the others. They must stand out and be more pronounced than the regular beat, hence the louder volume. Once the pressure is applied correctly, the volume rises to help strengthen the beat. Horns can apply pressure without making a sound. This occurs when the player's mouth isn't properly positioned on the mouthpiece.
There is no mystery in finding the accented beat. In musical notation, the time signature tells the number of beats per measure. Every first beat is accented. For example, the time signature denoted by a "4" above another "4" indicates that there are four beats to every measure and that the first beat of each set of four is accented, unless otherwise notated.
Vocalists provide further accent on beats in the tones used to enunciate their words. When the song is sung, the words sung on the accented beats are stressed or enunciated more than the ones sang normally. Parts of words may be accented over others, separated by syllables. Vocalists also use volume to accent words as notated on the sheet music.