How can you tell if its a sinus infection or flu? Also what are signs sinus infection has spread?

Flu vs sinusitis. Flu is characterized by sudden onset, fever, muscle aches. Sinusitis may cause sinus pain/pressure, post-nasal drainage, referred pain to back of head/neck; can be viral or bacterial in origin; fever suggests bacterial. Earache, painful swallowing, hoarseness, loss of voice suggest infxn has spread.
Sinus infections. Are localized to the sinus areas in the facial and nasal regions. Flu symptoms may be more generalized and systemic, accompanied by muscle and joint aches, upper respiratory symptoms including the ear and occasionally cardiac symptoms as well.