You’ll know it when you see it. Laid paper has that little ridge and furrow pattern built in to it.
Those furrows are made artificially nowadays, but the origin of the pattern goes back to the early days of papermaking, before we had invented machines to do the job. They weren’t as they now are a design feature; they were inherent to the process. Hand-made paper involves dipping a wire-bottomed sieve into a basin of water and pulp, pulling it out, and allowing the water to drain through leaving a sheet of (wet) paper behind. When you take the paper out of the mold, the pattern of the wires remains on the underside of the sheet: it’s a necessary consequence of the manufacturing process.
Nowadays so much of what we do in book manufacturing harks back to the old hand-work days: headbands, raised hubs on the spine, deckle edges, even the groove at each side of the spine, the half-title page, and lots of typesetting conventions. Nostalgia is alive and well in the book publishing business. When we decide to make a laid paper nowadays we do so for aesthetic reasons, and we have use a dandy roll, a light roller which carries the pattern and has no function other than to make an impression on one side of the web of paper. This pattern may include a watermark too: a laid paper doesn’t have to come with a watermark, just as a watermark can also be used with a wove (non laid, regular) paper.
The Oxford English Dictionary doesn’t really do justice to this term (this entry is unrevised since 1901). They include three paper-related quotations in a general definition of “laid”, all dating from the 19th century. Like so many of the terms we use now as if they were ancient, the descriptor only became a word once we had developed to a point where we could have alternative choices. They cop out in giving the meaning only as “Deliberately framed” which doesn’t really work for paper (or indeed corn, ice, stitches and many of the other instances they reference)!
This will only be of interest to me I dare say (though it’s the sort of fact my mother would have delighted in), but they also explain the origin of what I always thought of as “the mill lade”, a contained stream, a sort of mini-canal which ran through the middle of the ancient wool town in which I grew up and was once used to power all the mills built over it. A laid drain is apparently a channel lined with stones — an exact description of the mis-spelled lade.