Current loop Converter
There are several types of current loops- the old digital current loop which is serial, but they're seldom seen. 

There is analog current loop- typically (but not always) 4~20mA DC. 10~50 and 1~5mA are also seen. 

There is also analog-digital current loop with a base 4~20mA signal and a digital communication signal superimposed- eg HART (Highway Addressable Remote Transducer protocol). 

If it's analog 4~20mA, just put an ammeter in series. 4mA = minimum scale, 20mA  = maximum scale. 

If it's digital serial 4~20mA, you can put an optocoupler in series with some resistors to limit the LED current (and a diode across  the LED). You can then see the serial signal at the output. It's easier if you use a logic output optocoupler. 

If it's HART, you can read the analog signal as above, for the digital you need a modem so it would be best to get ahold of a proper HART interface.