Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for chest pain or pressure accompanied by shortness of breath, sweating, faintness, or pain spreading to the arm, jaw, back, or shoulder.
Do not assume chest discomfort is only trapped gas.
Vomiting blood, vomit that resembles coffee grounds, or black, tar-like stools also requires emergency care.
Seek same-day medical advice if belching comes with severe or worsening belly pain, repeated vomiting, a swollen belly that does not settle, or an inability to keep fluids down.
Arrange a routine appointment if belching is frequent, has changed without a clear reason, continues despite simple changes, or interferes with eating, sleep, work, or social activities.
You should also book an appointment if belching occurs with difficulty swallowing, frequent heartburn, ongoing upper-belly discomfort, reduced appetite, unexplained weight loss, or feeling full after only a small amount of food.
Every sign above is quoted from the sofidex article on Belching. If you are not sure, err upward — it is always reasonable to be checked.