My answer might not answer your question, but it seems to answer the intent behind it.
Let us consider two scenarios. The first is that the item is a +X item. The second is that it is a specific magic item. By the end let's consider the Shields, which are... a very special case.
It depends on the original item. Pages 144 and 145 from PHB state which armor has metal on it or not. This answer lists them. If you get a Breasplate +1, you can't use it.
Some of them are listed as "Armor (Plate)" or "Armor (Chain Mail)" - these are already implied having metal, so you can't use them.
If nothing indicates they are made of metal, neither in their type (e.g. (Chain Mail) as mentioned above) or in their description, they simply aren't, so you can use them. It's a similar concept to Spells do what they say. If the item doesn't state it is made of metal, it is not.
This is different from saying you can choose the material it is made of. It might be wood, wool or simply a magical thread that you don't even know what it is. What you know is: It's not made of metal, because it doesn't say it is made of metal, thus you can use it.
This one is delicate. The adventure giving you the loot should specify what kind of shield it is (Wood or Metal). If it doesn't, since both have the same AC, it doesn't make much sense to assume it is a metal shield just so druids can't use it.
About the problem you mentioned:
The aim of this question is just to avoid having my druid roll for an object that the druid would later discover he cannot use; there is a lack of time to get information when the time to roll comes
From my experience, agreed by Duck's comment1 , this scenario is very unlikely and probably simply won't happen. Either it will be specified by the item itself or by the adventure. If the DM doesn't say anything about it, ask him before rolling. "Is this item made of metal?". In AL, rewards are distributed at the end of a play session, as mentioned in the AL DM guide, page 5. So you should have time to search the DMG and check if the magic item is specified as a plate (for example, Armor of Vulnerability is) or what.
Also, if the item can't be used by you and you show interest in it, it's very likely that other players or the DM will warn you about it, mainly if they are experienced, and you will then (hopefully) change your decision.
1 I can't speak to every AL adventure but all the equipment that has dropped in the adventures I have done with my druid have specified material type. Which is frustrating since I use shields and have had to pass up 3 magic ones already (@Duck)