As everyone has said, use the checklist and fill it out for every stop on the journey back to Calais from wherever you’ve loaded.
Make a note of your seal number on your checklist as well. I thought we had a link to a .pdf checklist, but I can’t see it now.
Basically, it’s got spaces for your name, vehicle reg, trailer number, seal number; date, time, location of loading; date, time, location of check, etc.
(You were supposed to fill one form out for every check, I used one form per trip with a list of check places, dates, times and then ticked each check item several times.)
The rest of the sheet was a list of places on the vehicle to be checked, with a check box.
The unit:- under the chassis, inside your roof spoiler, inside your lockers ( 
), on your bunk(s) ( ), any other spaces where someone could conceivably hide.
The trailer:- under the chassis, on top of the axles, inside pallet carriers, on the roof ( 
), any visible damage to curtains, or TIR cord, or seal, any visible signs of some other way of getting into your trailer, any other ways someone could possibly hide themselves on your trailer.
Each trips check sheets should also be retained with any other paperwork appertaining to that trip. You can then show that you have always done your checks, you didn’t just knock one up for this one trip where you got caught with illegals on board.
It wasn’t written in stone that you would be fined for stowaways, only that you could be. Sometimes you’d “get away with it”, sometimes you wouldn’t.
I remember a letter in the Transport News paper, many years ago. A driver was in shock because he’d been fined for stowaways found on his trailer. The reason he was in shock was that he’d “got away with it” on three previous occasions and expected to carry on “getting away with it” 
Stowaways are not a road traffic offence, so there are no points on your licence for being caught with them on, just the possible fine. Up to £3,000 per head I think it is now.
If you are really suspicious that you’ve got stowaways on board, despite being vigilant, despite the Calais dock checks saying you’re clear, INSIST on them carrying out a Mk1 Eyeball check of your trailer, BEFORE you get to British Passport control. Once you reach that point, they’ll say you have entered the UK and have therefore carried illegals into the UK.
Despite that, if the illegals decide to just walk away, British Customs don’t have any authority to detain them because they’re on French soil. Work that one out, if you can.