If you have an ash tree that is infected there would have been leaf loss and die back of the crown of the tree prior to the autumn leaf fall. There would also have been black leaves on the tree prior to the seasonal change.
During the winter the main way of monitoring the disease is to look for lesions on the bark of the tree. The wood and pith underlying the affected bark is usually stained.
If you believe you have an affected tree then please check the Forestry Commission web site www.forestry.gov.uk for advice on what to do.
If you have a Mountain Ash (Rowan) tree, it is not at risk. It is not part of the ash family, and is so called simply because of a similar leaf shape.