Is there a method which checks to see if an entity can be seen? For example, if an entity is in the camera's view frustum and is not behind a wall or object return true.

I don't think there's a simple entity.canBeSee() function per say, but you have some options. You could do what the Enderman class does here

EnderMan.class

    boolean isLookingAtMe(Player p_32535_) {
        ItemStack itemstack = (ItemStack)p_32535_.getInventory().armor.get(3);
        if (ForgeHooks.shouldSuppressEnderManAnger(this, p_32535_, itemstack)) {
            return false;
        } else {
            Vec3 vec3 = p_32535_.getViewVector(1.0F).normalize();
            Vec3 vec31 = new Vec3(this.getX() - p_32535_.getX(), this.getEyeY() - p_32535_.getEyeY(), this.getZ() - p_32535_.getZ());
            double d0 = vec31.length();
            vec31 = vec31.normalize();
            double d1 = vec3.dot(vec31);
            return d1 > 1.0 - 0.025 / d0 ? p_32535_.hasLineOfSight(this) : false;
        }
    }

 

Maybe tho, this will fit your needs? `player.hasLineOfSight(entity)`

 

Alternatively you could do a raycast either using level.clip(...) or your own implementation using AABB's. A little more complicated but you have more control and tinkering power if needed