How to spot rail dust?

Short of going over the whole car using the baggie method, is there any easier way to spot contaminents (such as rail dust) on a new car painted a dark color? I figure it would be obvious on a light color car, but my new car is dark grey so I didnt think it would stand out very well.

Id like to determine if a claying of the whole car is necessary, if I can easily find the trouble spots, or if I can determine that claying might be totally unnecessary (although im not very confident about not needing it at all).

Thanks!!!
-Chris

The baggie is the best way but on new cars I always clay the whole car. It take 15 to 20 minutes and then you can be sure that its clean