First make sure the axis are all moving in the correct direction.
Next in Repetier Host send the M119 command in the G-Code command line.

Note the response in the text log:

When the endstops are not pressed they should read as Open
You can hold an endstop down and then send M119, it should then read Triggered if it is working properly. If you do not have this response then check that the endstop is plugged into the correct port and that the wiring is not damaged.
Ignore the Max endstops, we only use the Min endstops.
Also check that the X endstop relates to x_min and the same for Y and Z with the M119 command. If not then they are connected to the wrong locations.