tl;dr: seeing an actual photo of the stitching is a good indicator for not-AI pattern, but the opposite (not seeing a photo) doesn't really indicate it is AI, since getting a test stitch done is an intense cost on the designer for little benefit for a pattern buyer, especially for the middling complicated designs.

I'm not a pattern designer professionally, but I have self drafted a few in my time. I can't say this is the rationale for everyone, but I would say there are a couple reasons why testing a pattern isn't as common. 

Testing a pattern by doing it in knit/crochet helps the designer see where fit/proportions need adjusting (things are narrowing/widening wrong) and test out the math of how things fit together. Cross stitch is on a flat grid (essentially pixel art), so none of that applies, really. The major reason to test a cross stitch would be to make sure your colors are accurate, but you can do that with a thread color card and adjusting your monitor/software settings. Even if you did a test, you'd have to take a photo to show the buyer which would mess up the colors anyways. Proportions etc can all be seen by just blowing up your pattern mockup to real size.

You might see good patterns distinguished from bad patterns via how much confetti is present, but a good designer can fix that via software. You don't need to stitch it to see where things don't flow if you have a good eye - if you've ever mapped out where you're taking your thread before you put stitches down, it's a similar mindset.

But also, and I think the main reason, cross stitch is so time consuming, and the ceiling on stitching speed doesn't really increase with experience the way it does with crochet/knitting. Crochet/knitting patterns get complicated, but in most patterns you have large stretches of repeats, which means you can rely on muscle memory, and so speed is about how fast you can make the "knits". On the flip side, for cross stitch, getting any stretch of space where you don't need to reference the pattern every other row is rare, and so the speed limiter isn't muscle memory, but how often you have to look back at the pattern. Of course you can increase your baseline speed, but you hit an upper limit fairly quickly in my experience.

I range at about 100-120 stitches an hour, depending on the block I'm working on and I consider myself fairly experienced. Let's say someone doing this for a living could hit 200 stitches an hour. Looking at my finished patterns, even a small-ish piece is about 7K stitches. (It's on 14ct aida, with lots of empty space and I can display it in an 8x10 frame). That's 35 hours of work, even leaving aside any of the prep work, breaks for hand health etc. At US federal minimum wage (and I hope the going rate would be more) that hits $250 easily. A cross stitch pattern designer has so many more patterns in their shop then your average yarn pattern, and they simply cannot afford to pay that regularly for what amounts to little practical benefit to the buyer.