Finding Nessus False POsitives

[kaps lock <secnerdkaps () yahoo com>]

Hi All,
I would like to know how experieced vulnerability
assesment anaylsts determine nessus results as false
positives or not.
The way i ascertain a result being a false positive or
not is by crafting the same http request to the
webserver .
The point where i struggle is how to craft the same
request.I mean if nessus says 
"nessus was able to find the authentication mechanism
behind an smtp server as NTLM how cud i believe or
deduce tis true"
"or nessus cud actually upload a test file with PUT
and then delete the TEST file with DEL on the
webserver"

how can i determine these to be true?
thanks
kaps


                
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