Eelworms are microscopic pest - Nematodes - and not normally seen. In the case of the Strawberry Eelworm there is normally an elongation of the stems and or leaf stalks with red colouring, or may be stumpy short and thick. There is no treatment for eelworm, so dig up the infected plants and burn. Do not compost.
The reddening symptoms should not be confused with a disease of strawberries - Red Core - which is a fungal disease - not a pest.
Any doubt about the symptoms - appearance - should involve the burning off the plant affected, with no further planting in the area for at least 5 years.
It is also thought that Oat Straw can be a carrier of Strawberry Eelworm. Always use wheat straw.
Nematodes are normally minute worms - invisible to the human eye, and in most cases not treatable.