In the rules, there is no bonus for investigating an item for which you have the lab text. Essentially, you want to make sure that the item you have in front of you is the item you expect it to be. This could be useful if you have a secret cache where you are hiding an item for later use, and you are willing to spend a season every 20 years or so to make sure nobody found it and exchanged it for a nasty surprise. It could also be that your paranoid Tremere master charged you with checking that some contingency plan is still in working order. It could also be that you're looting a lab for some perfectly legal reason and you find a bunch of items along with some lab texts. (You can use an item variant of Sight of the Active Magics to check that the Form and Technique are correct, but once you verify that it's a CrIg or PeCo effect like it's supposed to be, do you really want to just try it?)
So let's say your InVi lab total is not quite high enough to find all powers, but you have the lab text used to create the item (you think). I feel this should get you a bonus which is not covered in the rules. What would that bonus be? Double your InVi lab total for the investigation? Make success automatic?
I think that is what an InVi specialist is for. Can't take away on of the few things they can do.
You can probably just use spont spells to validate the sigil is yours
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I could see a world where you would get +Mag of the Lab Text, as if it were a Known Spell being used to invent another - although you wouldn't know until you'd finished if what you were holding was doing anything for you, and I think that bonus should only apply if it's the proper thing (so, if you have Secondus' lab texts for a Knife of Wall Cutting, but you're investigating Quintus' Knife of Wall Cutting, you don't get the +Mag.) And it would probably have to be a thing only the storyguide knows for sure, otherwise it would give the game away.
If you have the lab text you have all the triggers, conditions, etc. of the enchanted item. Just use the item to see if it matches. Yes, this may be hazardous.
If you don't think it matches the lab text you shouldn't get a bonus because you don't trust the lab text is accurate.
No, using an item to find out whether it's your wand of BoAF or whether your enemy replaced it with a deadly trap is not an option.
In the case someone analyze the item and have the 'maybe' labtext - I would not give an bonus, if the labtest is the wrong one. Instead i would give the mage, soon after starting the research, the information, that the labtext is wrong - so he knows he search for undocumented effects