My novel starts with the character believing she is outside, naked, because she is delerious. After less than a page she realizes where she really is. She was delerious because she fainted and she is ill--but will it be seen as too much like a dream sequence?
Have you ever been drunk or had a high fever? It feels quite different from being in a dream.
When you dream, you feel normal, and only after you woke up may you realize that what you experienced was extraordinary, e.g. flying.
When you are delirious from a drug or illness, your body feels different and your perceptions are changed.
Whatever causes your character's delirium, all you need to do is research its physiological symptoms - such as nausea, dizziness or coordination problems - and describe how your character is having these.
If your character took a psychotropic drug, then the only symptoms may be hallucinations, and then there is no way to tell if the character is dreaming or delirious.