Trace to your point of interest (player or other enemy), if !trace_hit, calculate the angle from the direction of your entity to that object, for instance as follows:

c_trace(my.x,object.x,...);
if(!trace_hit)
{
vec_diff(temp,object.x,my.x);
vec_to_angle(temp2,temp);
// now compare the ANGLE temp2 to the my-orientation to check if the object is in the field of view of the my entity
if(abs(ang(my.pan-temp2.x)) < 90 && abs(ang(my.tilt-temp2.y)) < 50)
{
object is visible in front of my-entity
}
}

I hope this is what you wanted to do. Alternatively you can adapt the scan cone of the c_scan instruction (i.e. not use a full-sphere scan) and combine it with c_trace, then you should not have to compare any angles at all.

Regarding your second question, did you mean "mirror" instead of "window"? If it's only a window you want to ignore you could use the c_ignore feature.