One may wish that the Dwarves would not know war among themselves, making battlefield identification less important, but this is but a myth.  In comparison to humans, Dwarven strife is less frequent, but given the passions and pride of the Dwarves, strive is inevitable.

The primary means used for differentiation in battle (at least, among the Dwarves themselves) is the shapes of the dwarven's soldier's Shield, Helm and weapon.   The well-known preference of dwarves to hammers and axes is due largely to the ascendency of the clans that used those weapons as part of their signature. 

Beards are used primarily as personal identification and while may follow along with clan-traditions, has a much wider variance and is little used on the battlefield except among clans-mates.

Some common variances, in order of significance.

Helms:

Basic shape - Conical, round-top, flat top

Adornments Top-knots, Horns, Crests (front to back, side-to-side),Wings, Brims

Shields:

Basic Shapes - Rectangle, Oval, Roman, Round

Size 

Weapons:

Single/double headed

Flat, concave or convex striking surfaces. 

Top Spikes.

Armor:

Shoulder adornments have been used by some clans, such as long spikes on the Epaulette.  Armor is less frequently used due to little interest in providing grips for ones enemies or impeding fellow soldiers.

Boots

There have some instances where clans use such expedients as specially shod boots to provide a distinctive marching noise.  Typically only the most confident of clans uses this method, as it can greatly reduce their success at ambush.