Sometimes in early labor...but if in heavy labor - if you can tell she's visibly been pushing w/no kitten appearing - is a reason to get hold of a vet. Delivering a kitten and taking a break before contractions get going again and having more kittens, is not unusual.
One sign of pushing in my cats was a flicking of the whiskers straight forward (yes, straight forward), and they might make a "moof" sound as well, and you'd see the back arch, and a shudder of the muscles as they push. Hard contractions, even on a longhair, you should be able to see the uterus ball up and be hard - then relax.
If you have any doubts, you need to get hold of a vet, there could be a mal-presentation of a kitten, a very large kitten, or ?