My dentist suspects i have sinusitis, and that it is putting pressure on tooth #2. can he see my sinuses on the x-ray or is he not seeing decay?

Sinusitis: Yes, it is possible to see the sinus cavities on certain dental radiographs. And it is also possible for sinusitis to cause teeth to become symptomatic.

Sinus: The floor of the maxillary sinus is directly above the root of the upper teeth, so it is quite common for people with teeth problems to feel like they have a sinus problem and people with sinus problems to report tooth pain. The maxillary sinus cavities can be seen with the dental x-rays and if there is significant thickening or fluid in the sinus, it could indicate a sinus infection.

Can see sinus: The dentist can see the sinuses on a good xray. The sinus would appear cloudy (as compared to the other side). Sinus symptoms include a feeling of fullness behind your nose, or under your eyes. Pain that changes in intensity as your change your head position (bending forward causes more pain), tooth pain that really can't be traced to a single particular upper tooth.