You don't need to validate.
Just re-save it with computed shadows.
Alternatively, you can copy a bunch of TM1 Stadium tracks to your "My Maps" folder and then use a batch command to compute shadows on all of them at the same time. More info about that in this topic:
http://forum.maniaplanet.com/viewtopic.php?p=139476#p139476
How can you tell whether a map has lightmaps (has been converted for TM2) already or not?
One of the easiest ways is looking at the filesize. Maps with a lightmap included will be in the triple digits in KBs. Maps from TM1 without lightmaps included will be in the double digits in KBs.