Good point
It pretty easy to test
Where it's safe to do so
Drive at around 20 mph
Push the clutch pedal in and select 5th
Raise the revs to around 2.5K you'll have to guess as there's no rev counter in most actives, it's about what you would do for a hill start
When the road speed falls to below 10 mph release the clutch, if it judders and stalls it's fine,
If it keeps running then it's faulty, dip the clutch and end the test
Slipping clutch is normally first noticeable when accelerating in 5th especially up hill
There's a number of ways to check for a slipping clutch and the method you detail here will work as well as any. Personally - Oh Gawd, what's he going to come up with now? I hear you thinking

- I don't like stalling an engine so my usual way of doing it is to find somewhere safe to drive along in a higher gear, 4th or 5th works well, but I've also done it in 3rd although that can be less definitive. You need to be achieving around 2000 to 2500 rpm - it's not "super critical" - When there's plenty of room around you traffic wise, of course best if you can find a deserted bit of road, open the throttle wide, keep the throttle fully depressed so the engine is pulling hard and "dab" the clutch pedal. I mean by this that you are going to very quickly depress the clutch pedal to the floor and immediately, with no delay so you don't over rev the engine, let the pedal fully back up again. What should happen is that the revs will rise very quickly when you depress the pedal, and then, virtually instantly, come back down to what they were before - ie the 2000 to 2500 you started at. When you depress the pedal it's literally a very quick "Dab" during which the revs will probably rise to around 4000/4500 rpm - but again it doesn't matter, but don't hold the clutch pedal down so the engine over revs of course! I wouldn't expect the revs to exceed 5000 and I'd be happy enough if they didn't quite reach the 4000 I mention above. It's the fact the pedal is depressed enough to fully disengage the driven plate and allow the revs to rise noticeably that's important. The whole process is very quick and takes seconds only.
So what to expect? what you're particularly interested in is how the clutch "bites" when you release the pedal. If the clutch is good then the revs will rise when the pedal is depressed but immediately return to where they started when the pedal returns. So you'll hear, and see on the rev counter, the instant increase in revs as you dip the clutch followed immediately by a return to the previous revs as the pedal is fully released again. If you have a good "ear" you'll hear it without needing to look at the rev counter. If the clutch is slipping badly the revs will not reduce when the pedal is released - indeed, if the clutch is truly on it's last legs, the revs may go on increasing so you'll need to be ready to quickly take your foot off the throttle to avoid over revving! There are also lesser effects if the clutch is just a bit "iffy", which mostly consist of a gradual reduction of revs once the pedal is released rather than a proper immediate "bite" instantly returning the revs to the previous 2000 starting point.
Of course, whatever procedure you decide to use, these "stress tests" put the clutch under extreme duress so it's only something you'd be doing if you strongly suspect there's a problem. I've tested many vehicles in this way and I like it because, apart from refraining from doing it in heavy traffic, you can do it just about anywhere. Oh, and by the way, if the clutch is slipping, don't be surprised if inducing slippage in this extreme way results in a disgusting smell of burnt lining!