The official website from the EU Commission mentions several criteria:
- The commercial status of the holder of the products and his reasons for holding them;
- The place where the products are located or, if appropriate, the mode of transport used;
- Any document relating to the products;
- The nature of the products;
- The quantity of the products.
There is always some judgment involved and generalising from individual experience might therefore not be very useful but if you really need to carry more than 90 litres of wine for your personal use across a border, then you presumably would be able to explain why. I suspect the question is puzzling because you do not really need 90 litres of wine for your personal use. As far as documentary evidence goes, anything could help (evidence of past consumption, evidence that you are a collector, evidence you have enough money to afford so much wine without reselling it at some point, etc.) but a random check is not necessarily very likely.
If you read between the lines, you can see the quantities are quite generous (compared to what you can carry between just about any other pair of countries, especially for alcohol) and the criteria are designed to provide cover to the police in obvious cases. That's purely a guess but I suspect that if you come back from Scotland with twenty bottles of whisky, carefully packed in your trunk, all of them different, you should be able to argue they are for your personal use successfully. If you have thousands of identical cigarettes in some hidden compartment in a truck, you can't turn around and somehow prove they are for “personal use” if you get caught because it's obvious you were trying to smuggle them.