Tricks for spotting grass strips in grass fields:-
1) The field has to be long enough for a runway. So that eliminates a number of them.
2) It's got to be accessible by road, so look for a road on one side of the field.
3) There's likely to be some sort of building on one side of the field, probably hangar shaped/sized.
4) There's likely to be at least one or two other aircraft, if you look closely.
5) A runway is likely to be of shorter grass than the rest of the field, although at this time of year that isn't always the case. And farmers do have a habit of mowing strips of grass too. BUT you have to be able to get to both ends of the runway, or turn round at one end and backtrack, so look for taxiways or turning circles...farmers don't do them.
6) Finally...look for a windsock.