Hi Iain
If you hold the hen, backside pointing forward and feel about an inch below her vent, you should feel a pair of sharp(ish) small bones just below the skin (we call these pin bones around here). If the two bones have a gap the width of your finger, or larger the chicken should be laying - the bones need to be apart to let the egg out. If the two bones are very close together then the hen isn't laying.
Most hens will lay provided they have light (14+ hours per day), shelter and a nestbox, water and feed/minerals. However even small events (e.g. a low flying plane, thunder storm, different people handling them, etc) can lead to hens deciding not to.
Regards
Alan