PIR wiring load and switched live

[cliverb]

Hi, apologies for the daft question....
I am replacing a PIR sensor connected to a number of external lights and (very) stupidly didn't take a picture of the wiring before disconnecting it. I have two red cables which I assume are load and switched live which go into separate junction block slots and two black neutral cables which are slotted into the same block slot. The cable coming out the wall which was connected to the old PIR also has an earth but I don't think this was wired up to the PIR and the new PIR doesn't have an earth.
I can see from the PIR paperwork that I wire the neutral to neutral but I don't know which existing red wires are the switch live verses load....
My question is if I wire it and get it the wrong way round what is likely to happen - just trip the fuse switch in the consumer unit, just not work or blow the new PIR sensor?
Not sure how I would determine which is switched live verses load?
Any help gratefully received thanks!

 

[Sureitsoff?]

Assuming one of the reds is the perm live and the other the SW live, the one that is the perm live will "power" the PIR itself . I don't think getting them the wrong way round will kill the PIR as the perm live feeding " the wrong way" into the PIR will be stopped by the switching of the pir

 

[cliverb]

Thanks that makes sense cheers.