My guitar amp has a FX loop, but I don't know if it is serial or parallel?
On most amps, you could apply the simple folowing test
Take a jack cable and insert it in the return of the loop. Play on your guitar:
If you do not get any signal in the output, you have a serial loop
If you can still hear your guitar, it is because you have a parallel loop.
In this case the amp will have a mix control to adjust balance between direct path and loop return (Dry/wet)
On many amps setting this control to 100% (maximum) will turn the loop into serial. Then you should experience option 1. (no signal in the output).