I live in a small village and there is a lovely walking route with nice dog friendly stiles and good signposting.  A farmer has place a wooden hand painted sign saying "No Dogs" on the stile.  This is on there all year round even when there are no livestock in the field.  Im just curious as to if this is legal ?

Are you in Scotland or England?  As there are a couple of different laws.  I think it is ok to "trespass" as long as your dog is on a lead.

I think it depends what kind of path it is.  There is some info here
http://www.ramblers.org.uk/info/britain/access.html#Dogs

It's important to know exactly what Right of Way it is - check on an OS map whether it's classed as a footpath.  Dogs are allowed on here and the ramblers site you've been given is a useful one.

Your Parish Council will be your best option for tackling the problem, and it's likely they will have received more than one complaint if the farmer is in the wrong.  Most PC's will have some kind of relationship with the local farmers over issues such as hedge cutting and maintenance of styles and gates etc so they may just be able to handle it with a phone call.

There was a huge thread on here recently about livestock in fields with Rights of Way.  The farmer may have put the sign there whilst he had lambs or calves and just 'forgot' to remove it, or he may know that locals know better but is hoping to dissuade ramblers!!