When you say underlayment… I assume you mean felt paper or similar. I’ve stepped thru the concrete tiles when there is skip sheathing under it, hope you didn’t.
Anyway, if I looked at a concrete tile roof with no felt or building paper under it, I would certainly recommend in my report to have a roofing contractor evaluate.
You can find a handful of resources out there.
Basically, a concrete tile sheds water, but the underlayment provides the actual moisture barrier. As a matter of fact, I let my clients know that I can not see the underlayment/moisture barrier (most are not skip sheathed) while performing a roof inspection.
If this not what you meant, the address I gave you should provide some info