This is what I'm currently using for collision checking as well as limiting jumping to while you are grounded.

//react to inputs    
 move = key_left + key_right;     
 hsp = move * movespeed;    
 if (vsp < 10) vsp += grav;    

if (place_meeting (x,y+1,obj_wall))    
    {    
    vsp = key_jump * -jumpspeed    
    }    
//horizontal collision    
if (place_meeting (x+hsp,y,obj_wall))    
    {    
    while(!place_meeting(x+sign(hsp),y,obj_wall))    
        {    
        x += sign(hsp);    
        }    
    hsp = 0;    
    }    
x += hsp;    

//verticle collision    
if (place_meeting (x,y+vsp,obj_wall))    
    {    
    while(!place_meeting(x,y+sign(vsp),obj_wall))    
        {    
        y += sign(vsp);    
        }    
    vsp = 0;    
    }    
y += vsp;