The alarm I have has the option to detach the sensor from the alarm box connected by a long built-in wire.
I have the choice of setting the alarm on the floor but, my thought was that I wanted to be notified BEFORE the crock level overflowed onto the floor.
So I detached the sensor, let the crock fill to normal level just before the sump kicked on and set the sensors about an inch above the highest water level in the crock.
If the sump fails, the sensor will alarm shortly after the water level goes beyond normal height and I can start bailing.
As a backup, I installed two sumps in the crock. The secondary pump has it's float level set just a bit above the primary. So when I hear the alarm, I know I have a real problem.
Here's a link to one of the types with a wired sensor.