Use a for/next loop going from every object in your game, then cast an intersection ray below the players object downwards x amount of units. If an intersection occurs with the current object, some part of it is directly below your player. Pseudo;
FOR x = 1 TO 10
IF OBJECT EXIST(x)
C# = INTERSECT OBJECT(below user)
IF C# > 0
TEXT 0,x*15,"Object "+STR$(x)+" is below you"
ENDIF
ENDIF
NEXT x
Or use Sparky's (it's faster). If you use Sparky's, you can assign all of your objects to a group, then just perform one intersection check with that group, and it will return which object from the group is intersected. Would be faster/less lines of code.
Projects: Online CTF Game | Newcommer's Guide to FPS's