first of all get your bounding boxes right, try making a red cube that is semi transparent to show your bounding box, this will help you see if it's correct. Then if you have a propper box, a simple pure maths way of doing the check is by checking of the players x,y,z is within every boundary (a box has 6 boundaries, you could call em: xstart,xend,ystart etc.) so if the x > xStart and the x < xEnd, the y > yStart etc. then the players current position is in the box.
However the player will no doubt have a radius, to compensate for that you need to expand the box by the radius, like this: if the x > (xStart-radius) and the x < (xEnd+radius), the y > (yStart-radius) etc. then the players current radius is, atleast partialy, inside the bounding box. Hope that helps ya. You really want to shy away from using entityvisible for stuff like that it's a bit slow.
However the player will no doubt have a radius, to compensate for that you need to expand the box by the radius, like this: if the x > (xStart-radius) and the x < (xEnd+radius), the y > (yStart-radius) etc. then the players current radius is, atleast partialy, inside the bounding box. Hope that helps ya. You really want to shy away from using entityvisible for stuff like that it's a bit slow.