Always worth having a look under the carpet in the near-side footwell to see if there are signs of it having been fitted with dual controls. If it had, the carpet itself may have been changed and may look appreciably newer / cleaner than the drivers side.

A patch on the roof where the plastic film was fitted to allow the roof sign to be taken on and off safely is the only other give away I can think of.

When I sold my SOM car (a Focus) the holes for the dual pedals had been filled and the carpet repaired very effectively, but a careful inspection would definitely have given the game away.

The big school (BSM) uses Fiats now and they sell them on after 6 months to a year via approved used schemes etc. The main ones using Micras are smaller independant operators ho will probably sell the car privately to a client or get shot via Autotrader when it's covered about 100K.

Ultimately though, why worry about it? The chances that you'll get an SOM car are very slim, only a tiny proportion of Nissan Micras will have been used in such a way. Corsas are a different story though as that's what BSM used until very recently.