In my last game, we used an alternative to occlusion queries for this problem, but it requires that you're able to bind your depth-buffer as a readable texture -- I don't know if XNA allows that, but it's possible in D3D9, so maybe.

First we calculate on the CPU the expected depth value of the light source, and it's screen space position:
N.B. all untested psuedo-code as example

Vector4 lightData = Vector4( light_position, 1 );//get the light's world position, make sure that 'w' is 1.0
lightData = ViewProjectionMatrix * lightData;//multiply with the view-projection matrix
lightData = lightData / lightData.w;//perform perspective division
lightData.x = lightData.x*0.5+0.5;//the x/y screen coordinates come out between -1 and 1, so
lightData.y = lightData.y*0.5+0.5;//we need to rescale them to be 0 to 1 tex-coords

Next we render a quad that completely covers a small 16x16 pixel render-target. The quad should have UV's from [0,0] to [1,1] like a regular full-screen quad would. We also bind the depth-buffer as a texture, and put the above lightData in a shader constant.
In the vertex shader, assuming the input UV's are from [0,0] to [1,1], we can calculate the output UVs like:

float2 pixel = float2( 1/1280.0, 1/720.0 );//adjust depending on the resolution of your depth buffer!
Out.UV = lightData.xy + (In.UV - 0.5) * pixel*16;//the quad will take depth samples in a 16x16px region of the depth buffer, centred around the light position

In the pixel shader, we can then test each depth-buffer value against our predicted light depth, and output the result of the depth test as white or black.

float depth = tex2D( depthBuffer, In.UV ).r;
float test = step( lightData.z, depth ); // test = lightData.z < depth ? 1.0 : 0.0
return test.xxxx;

Now the 16x16 render-target contains 256 boolean values, indicating whether the light is in front or behind that pixel nearby it.
If we now generate mip-maps for this render-target, then mip-level #4 will be a 1x1 texture, containing the percentage of visible pixels.

In your actual lens flare shader, you can sample this texture to find out how much to scale/fade the flare effect, e.g.

float visibility = tex2Dlod( occlusionBuffer, float4(0.5,0.5,4,0) ).r;

In my last game, we used an alternative to occlusion queries for this problem, but it requires that you're able to bind your depth-buffer as a readable texture -- I don't know if XNA allows that, but it's possible in D3D9, so maybe.

First we calculate on the CPU the expected depth value of the light source, and it's screen space position:

Vector4 lightData = Vector4( light_position, 1 );//get the light's world position, make sure that 'w' is 1.0
light = ViewProjectionMatrix * lightData;//multiply with the view-projection matrix
lightData = lightData / lightData.w;//perform perspective division
lightData.x = lightData.x*0.5+0.5;//the x/y screen coordinates come out between -1 and 1, so
lightData.y = lightData.y*0.5+0.5;//we need to rescale them to be 0 to 1 tex-coords

Next we render a quad that completely covers a small 16x16 pixel render-target. The quad should have UV's from [0,0] to [1,1] like a regular full-screen quad would. We also bind the depth-buffer as a texture, and put the above lightData in a shader constant.
In the vertex shader, assuming the input UV's are from [0,0] to [1,1], we can calculate the output UVs like:

float2 pixel = float2( 1/1280.0, 1/720.0 );//adjust depending on the resolution of your depth buffer!
Out.UV = lightData.xy + (In.UV - 0.5) * pixel*16;//the quad will take depth samples in a 16x16px region of the depth buffer, centred around the light position

In the pixel shader, we can then test each depth-buffer value against our predicted light depth, and output the result of the depth test as white or black.

float depth = tex2D( depthBuffer, In.UV ).r;
float test = step( lightData.z, depth ); // test = lightData.z < depth ? 1.0 : 0.0
return test.xxxx;

Now the 16x16 render-target contains 256 boolean values, indicating whether the light is in front or behind that pixel nearby it.
If we now generate mip-maps for this render-target, then mip-level #4 will be a 1x1 texture, containing the percentage of visible pixels.

In your actual lens flare shader, you can sample this texture to find out how much to scale/fade the flare effect, e.g.

float visibility = tex2Dlod( occlusionBuffer, float4(0.5,0.5,4,0) ).r;