Leave the A928 just before Kirriemuir and take the un-numbered road leading west. About a mile and a half along this you will see Lendrick Lodge on your right (north of the road). If you park here carefully, then there's a gate into the field south of the road, where the stone lies.
The lady who lives in the Lodge is very informative. She told me a bit about the stone. Apparently a good few years ago the farmer at Linross was one of these 'progressive' types. The stone was removed and dumped in a corner of the field. A short while later, a man with a clipboard turns up at her door, asking 'didn't there used to be a stone in that field', so she told him the story. Two weeks later the stone was back in its original position.... There are no noticable distinguishing marks on the stone.