If the screen doesn't end up being flat, you will get distortion in the projected image. If the wall is crooked and you pull the screen tight against the wall, the screen will be crooked and the image will be distorted. So the first thing to figure out is whether the screen is flat and the wall is crooked, or the wall is flat and the screen is crooked.
Pull the screen down and try holding it against other walls in the house. If you try two or three different placements and the screen doesn't lie flat in any of them, it's a pretty good sign that the screen is crooked.
If you want to check the wall directly, you could use a plumb line. Hang the line from the ceiling close to the wall at one end of the screen. When the line comes to rest, measure the distance from the wall to the line at the top and again at the bottom. Move the plumb line to the other end of the screen and repeat the measurements. If the wall is flat, the measurements should be more-or-less the same from one end of the screen to the other.
If the wall is crooked, you could remove some drywall and shim out the offending studs, then patch the drywall and repaint. A lot of work, but it would be fixed. If you don't want to tear into the drywall and you don't want to live with the shadow, you could try putting some black velvet on the wall around the screen. Chances are that would hide the shadow.
If the screen is crooked, you might be able to fix it by removing and retensioning the screen fabric.