Is there a way to put a relay in series with another load?
If you have a fully-enclosed device with internal "float sensor" and don't have access to internal load, the option is to sense load current when it gets active.

There are many ways to do this.

One simple method is to use a shunt resistor (10-20 mOhms for 6A range) and use a specialized IC like ZXCT1009 or similar, 


Then use a comparator on Vout to trigger an alarm. The whole circuit could be designed as independent self-contained stand-alone box, Vin - To-load. 

Alternatively you can use something like "Current Sensing Relay Switch" if you are willing to spend $70 or something. This is for AC sensing, but anyway:

  

There are current-sensing switches of industrial control grade, even maybe in a ready-to-go form. Many options are available if searching for right terms.