Inspired by the Rohloff HG-IG-Check tool (HG-IG-Check instructions pdf →) I got an idea (certainly not new) that I would call the poor man sprocket wear calliper (a man not too poor, as he needs a new chain).
What follows is more or less my post to Brompton-Talk in October 2009.
0) unfold the bike and lean it against a wall
1) remove the chain tensioner
2) install the new open chain so that it hangs down the rear of the sprocket and the front of the chainring
3) with the chain on the sprocket to be checked put the cranks so that the right crank is horizontal and forward
4) standing at the right of the bike and applying the rear brake, push with the right foot down on the crank to put the chain under tension. You should aim at about 50 kgf of tension in the chain, for which 40 kgf on the pedal (or less for smaller than 54T chainrings) should be enough.
5) with the left hand reach for the third engaged link and flip it in and out of the sprocket. If the link does move freely the sprocket is fine. If the link tends to engage in the tooth profile the sprocket is worn.
When I tried on a 3000 Km Brompton 2 speed I noticed that the sprockets (12T and 16T) were at the limit, I could feel a slight engagement, but veeery slight. A new sprocket should leave the third link completely free.
This test is easier when on the smaller sprocket as there is more free chain to play with. Removing the openable link of course makes it easy even for the larger sprocket.
Some help from a second person pushing on the crank and applying the brake would actually allow a higher chain tension.
Just checked with an identical B still on its original SRAM PC10 and sprockets and the third link (on the small sprocket) moves very freely. Clearly a chain and sprocket wear together well...
So, if your new chain sticks on the sprocket it's better to wait before replacing the chain, and replace chain and sprocket together.