There are many reasons an earth based dam or pond could be leaking. Soil type, age & poor contruction can all cause a dam to leak. The dam can also leak in different areas.
A wet area forming close by the outside of the dam is a good indicator the dam is leaking. Water will escape through or under the dam wall & pool nearby.
If the water level is dropping but no wet area has formed it may be leaking from the base or simply be evaporation. Take note of the decreasing water level. If it is larger than expected due to evaporation it will likely be leaking from the base.