An overwatered and underwatered plant can look very similar - dying leaves. However, generally the brown dead leaves of an underwatered plant will feel crispy, whilst the brown dead leaves of an overwatered plant will still feel limp.
So, when to water? To work out it you need to water, poke your finger a few centimetres into the soil. If it's damp you don't need to water. If you water too much, the soil will not have enough oxygen in it for the roots to breath.
Binley has some good detailed information about saving overwatered plants.
