I'm not exactly sure what you are asking...Is there any standard stating how to interpret this type of callout if the degrees are not specified? 15’ would be the same as ????°.
Engineering drawings are interpreted to whatever standard is specified on the engineering drawing. If the engineering drawing does not have an "Interpret per. Standard # ...." statement then anything goes.
If you have an "anything goes" engineering drawing I suppose 15' (minutes) is known to be .25° by tribal knowledge and known math syntax.
The point of having standards specified on engineering drawings to get everybody on the same page in terms of interpretation as well as drawing drafting practices consistency.