How can you check? Make a backup copy of a couple of production domain controllers, restore it to a test environment that is not accessible to/from your production Active Directory network (important), perform the update (adprep and dcpromo), and perform your validations. This could be as simple as setting up vms on your notebook. Suffice it to say that you would eventually need to do have an offline test environment anyway before proceeding with a production upgrade.
If there was something installed that updated the schema that you want to remove, you usually are aware of it, as there may be attributes on the various objects (User/Group/Computer/OU) that are no longer required.
One driver for going for a clean installation is the existing directory has undergone a period of "unstructured evolution", where there has been changes that were not documented properly. Another driver is if you cannot risk anything going sideways with the update and cannot backout.
One area of concern when adding the first uplevel domain controller may be if you have a large directory and/or a lot of domains, whenever you perform a schema update (using adprep /forestprep), all of the global catalog partitions are rebuilt. This may take a very long time. This also applies to other large schema updates such as for Exchange.
If you have an existing domain that started as Windows 2003 and have not yet enabled strict replication, you should enable that now, to surface any bogus objects in the directory, so it is not associated with the upgrade. You can read more about that here:
Enable strict replication consistency
http://technet.microsoft.com/en-us/library/cc784245%28v=ws.10%29.aspx
If there are lingering objects in the directory, enabling strict may cause replication to stop, so those objects need to be dealt with before any upgrade. If there are a lot of these objects surfaced, it may be time to consider the clean installation approach.
