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 if tongue swelling is rapid or you have trouble breathing.
Other emergency warning signs include throat tightness, inability to swallow saliva, faintness, widespread hives, or swelling of the lips, face, or neck.
These may indicate a serious allergic reaction.
Seek prompt medical care if swelling is worsening, you cannot drink enough, or you have fever with severe mouth pain or spreading swelling.
Prompt advice is also important if you have a weakened immune system and develop new mouth inflammation or signs of infection.
Arrange a routine dental or medical appointment if symptoms last longer than about two weeks, keep returning, or interfere with eating.
Seek assessment if glossitis occurs with tiredness, pale skin, unexplained weight loss, bowel symptoms, numbness, or tingling.
The article also says
A tongue ulcer, lump, hard area, or unexplained red or white patch that does not heal should be examined.
These findings often have non-cancerous causes but should not be assumed to be glossitis.
Every sign above is quoted from the sofidex article on Glossitis. If you are not sure, err upward — it is always reasonable to be checked.