In the movie we can see that Weasleys' Wizard Wheezes shop is big and packed with people and things. There are just two of them, they can't see everything. Supposedly they won't have cameras or anti-theft gates like Muggle shops do. Are there some kind of spells placed on the shop so that they would know if someone didn't pay?
Certain spells are used to stop theft.
Dumbledore warns readers not to read Quidditch Through the Ages without buying it for too long, otherwise they're going to experience a Thief's Curse.
“By buying this book — and I would advise you to buy it, because if you read it too long without handing over money you will find yourself the object of a Thief’s Curse — you too will be contributing to this magical mission.” - Quidditch Through the Ages
Fred and George could have placed the Thief's Curse or a similar spell on their products. In addition, there could be other spells to prevent theft that were just never mentioned in the story. Stopping theft with the aid of magic would likely be fairly easy, since theft would be a common problem for shop owners, and they've likely developed spells or charms to use against it.
In addition, Fred and George did have staff - it wasn't just the two of them.
Fred and George were the owners of Weasleys' Wizarding Wheezes, but they weren't the only staff. We see at least one other staff member.
“A young witch with short blonde hair poked her head round the curtain; Harry saw that she too was wearing magenta staff robes.
‘There’s a customer out here looking for a joke cauldron, Mr Weasley and Mr Weasley,’ she said.” - Harry Potter and the Half-Blood Prince, Chapter 6 (Draco's Detour)
From Harry Potter and The Half Blood Prince, chapter 3, "Draco's Detour", it is clear that Fred and George keep an eye on the goings-on of the customers in the shop, as muggle shopkeepers would do.
" That's three Galleons, nine Sickles, and a Knut,” said Fred, examining the many boxes in Ron’s arms. “Cough up.”
"I’m your brother!”
“And that’s our stuff you’re nicking. Three Galleons, nine Sickles. I’ll knock off the Knut.”
“But I haven’t got three Galleons, nine Sickles!”
"You’d better put it back then, and mind you put it on the right shelves.”
Also, earlier in the chapter:
"Giving him the tour? Come through the back, Harry, that’s where we’re making the real money—pocket anything, you, and you’ll pay in more than Galleons!” he added warningly to a small boy who hastily whipped his hand out of the tub labeled EDIBLE DARK MARKS—THEY’LL MAKE ANYONE SICK!"
So, presumably, this displays the presence of anti-theft curses and/or spells that could curse anyone who stole merchandise.