Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency medical help for possible pulmonary embolism, especially:
A newborn with rapidly developing purple, dark red or black skin patches needs emergency assessment.
Emergency assessment is especially important after a head injury.
Get urgent same-day medical advice for new one-sided leg swelling, pain, warmth or skin colour change.
These symptoms do not always mean DVT, but they need prompt assessment.
If you take an anticoagulant, seek urgent help for significant bleeding, vomiting or coughing blood, black stools, severe unexplained headache, or a major injury.
Routine follow-up is appropriate for treatment reviews, pregnancy planning, questions about contraception or hormone therapy, and planning before surgery or long travel.
Also arrange a review if ongoing leg swelling or breathlessness continues after a treated clot.
Every sign above is quoted from the sofidex article on Congenital protein C or S deficiency. If you are not sure, err upward — it is always reasonable to be checked.