First off, try looking at the reflection of light on the surface of the decal. Chances are the locating holes have made small dimples
You can get a rough idea of where they are by using the locating holes in the upper wing as a guide, put it alongside and you'll find the approximate area
Spars often stood on ribs, so that will help too
If you have to go probing for holes, use a fine sewing needle, the pin pricks from such a fine point won't show up and can easily be filled with a bit of varnish
And next time, do one wing at a time, then use the other wing to locate the holes you just covered
Finally, most modelling includes a lot of 'how do I get out of this mess?'. When you've got out of a lot of holes you too will be able to offer advice - guess how we all learned...
Richard