Well, there is no way to tell exactly, but there are a couple of guidelines.
Feel the breastbone of the hen - if it feels very flexible, then it is a pullet. If it feels brittle but still has some give, guess the age at around a year. If it is unyielding, then it is an older hen.
I'm not sure how to describe this, but at about two years, my hens always get the "big butt" look.
Another way is to look at the legs - an older hen's scales will be larger and less smooth. Of course, don't confuse this with a younger bird that may be infected with leg mites!
I'm not sure how you tell, but I do know that I have some fairly old gals! The first ones I bought were from a garage sale about 5 years ago, and that same year my cousin gave me some girls that were 2 or 3 years old and I still have 2 of them. Of course one of them has only laid 1 egg a year since I have had her, but I keep her because she is beautiful! I wouldn't want her to overdo it and go for......oh lets say even 2 eggs each year!
I thought about cutting her legs in half to count the rings but I've done that before on another and it is hard to find a good glue that works to put them back when you are done.
On a serious note...I have a Silkie that is finally recovering from her abuse and mites. Her feathers are actually starting to fluff out and now she looks like a chicken instead of a rat w/wings. BUT-no eggs, barely a comb and I just can't figure out how old the bird is.