At least it's people pocketing things. I used to work in a large warehouse-like store with a garden centre (not naming type/name of store), and people would routinely take very expensive items and pass them over the garden centre fence into car boots.
Alternatively, staff would fill a flatbed trolley with high-value goods, leave it at the exit with a note saying, "Order for Mr. Whatever", mark it as paid, and his mate would come and collect it for free.
Finally, since staff knew where the dummy cameras were, there were a couple who would take goods under a counter, swap them around, and buy what appears to be a very cheap box of something with something worth a fortune inside.
But in terms of advice, it's mainly company policy. You can't assume anybody is shoplifting until they have left the shop, as they may actually be using their pockets to carry things round the shop. Likewise, you have no real right to frisk somebody.
You can however ask people to empty their pockets, and if they don't, then call the police. You can also place people under citizen arrest, but that depends how confident you are!