Can you identify this Cateye HR Sensor E product model?
It is not ANT+ - only the HR-11 is listed on "thisisant.com" (and its too old to be ANT+) 

I suspect it will analogue - most likely would be Polar compatible but cannot be certain. There are Cateye digital monitors, and also ANT+ monitors, the digital ones have "Digital" written on them, which is why I suspect analogue 

Best option (IMHO) would be to turf it and go ANT+... The other go down to a local gym, or find a someone with Polar compatible gear and see if their gear picks it up (put in a new battery). But what will you do if you can find this out - buy a new watch based on obsolete technology?