Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Get urgent public health or emergency advice if:
Call emergency services for severe breathing difficulty, collapse, confusion, a seizure or difficulty waking.
Mention any suspected smallpox exposure during the call.
Do not use public transport or enter a crowded waiting room unless emergency staff direct you to do so.
Smallpox is not a routine explanation for fever or a rash.
Chickenpox, shingles, mpox, hand-foot-and-mouth disease and medication reactions are far more likely.
Seek routine medical advice if you have a new unexplained rash, particularly if it comes with fever or you feel unwell.
Contact the service before attending if the rash is blistering or you may have an infectious illness.
Explain the possible exposure clearly so staff can arrange safe assessment.
The article also says
Call before going to a healthcare setting.
Every sign above is quoted from the sofidex article on Smallpox. If you are not sure, err upward — it is always reasonable to be checked.