Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Treat any suspected yew ingestion as an emergency, even if there are no symptoms yet.
Call your local emergency number or poison centre immediately.
In the United States, Poison Control is available at 1-800-222-1222.
Call emergency services now if the person has:
If the person is unconscious and not breathing normally, begin cardiopulmonary resuscitation if you are able and follow the emergency operator’s instructions.
And for anyone, someone who:
Seek urgent reassessment for new fainting, breathing difficulty, chest symptoms, or an abnormal heartbeat.
After hospital treatment, attend planned follow-up appointments.
The article also says
For possible exposure without swallowing, such as touching a hedge, wash the skin with soap and water.
Rinse exposed eyes with clean lukewarm water.
Seek poison-centre or medical advice if plant material entered the mouth, symptoms appear, or the type of exposure is uncertain.
Every sign above is quoted from the sofidex article on Yew poisoning. If you are not sure, err upward — it is always reasonable to be checked.