Common signs of a Ant infestation include:

.Live adults: The most common ant to cause problems here in Cumbria tends to be the Common Black Garden Ant (Lasius Niger).  Workers are around 5mm in length with queens up to 15mm. Mating flights are often visible mid to late summer as winged male and virgin queens mate. Another less common ant here in Cumbria due to the cooler climate is the Pharoah's ant. They prefer tropical conditions and so are usually found inside warm buildings. Workers are around 2mm in length and are pale brown with a darker abdomen.
. Sightings in workplace or home: The Common Black Garden Ants tend to nest outside in loose soil and under paving. Trails can often be seen leading from these nests to a food source. Poorly stored food items and bad house keeping can provide an easy food source for the ants. Once the food source is found, a few ants can quickly turn into many!