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 care if the toes below a harness or cast become blue, very pale, cold, or markedly swollen, or if the child cannot move them.
Follow up promptly if your baby was breech or has a close family history of DDH and recommended screening has not been completed.
If a child is wearing a harness or cast, contact the treatment team the same day about skin sores, a harness that slips or no longer fits, a wet or cracked cast, worsening pain, fever, or a bad smell from inside the cast.
Severe or rapidly worsening pain also needs urgent assessment.
These signs may indicate pressure on circulation or nerves.
Arrange a prompt, non-emergency appointment if a baby moves one leg less, has difficulty opening one hip during routine care, or appears to have unequal leg lengths.
A new limp, waddling walk, or persistent hip or groin pain also needs assessment.
The article also says
DDH itself is not usually an emergency, but early assessment matters because treatment is often simpler in younger babies.
Every sign above is quoted from the sofidex article on Developmental dysplasia of the hip. If you are not sure, err upward — it is always reasonable to be checked.