Given that the common dog-rose can often occur in white it would be wrong to say the the easiest way to identify the field rose (Rosa arvensis) is to look for a white wild rose! Quite often white roses in the countryside will be field rose and it is a good start but it is not enough on its own to be sure.
When in flower the recognised botanical way of separating them is to look at the centre of the flower amongst the orange stamens and there is a column longer than the stamens (this is the style) then it is field rose. Less scientific methods can be used, however, as, whilst similar, they are not the same! The field rose tends to be a low, sprawling bush growing to little more than three feet tall as it has quite weak stems and this leads to its other common name, the trailing rose. The dog-rose, on the other hand, is a much stronger plant that produces long, prickly runners. At a closer level the leaves of field rose are a dull green where are dog-rose is a much stronger dark green on the top and a greyish green below. Field rose is also less prickly than the dog rose.There are many other small differences if you feel inclined to examine the two species with a good book at your side.
Whilst both occur in hedgerows the dog-rose is the better climber. Both can also be found in woodland or scrub habitats.