You can check the resistance values of both the primary and secondary winding to see if they are within spec. The correct values would be found in the manual, or perhaps doing a search here might yield something. In my limited experience I have found that coils *tend* to be either good or bad. I'm sure "weak" coils probably occur, but I've never seen one.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280