While this is doable in several genealogy packages I know of no way to do it when it comes to online trees.
The only thing I can think of is maybe varying the person's name so you can tell at a glance if they are completed.
If a man (John Doe) has a complete record you could change his name to Mr John Doe.
Likewise for a female (Mary Doe) you could change her name to Miss Mary Doe.
That way the index could easily tell you who is completed and who isn't.
I'll admit, it is a kludgy workaround but is all I can think of at the moment.