If you have seen my Discord message, then this is the same thing. From many people who have accidentally lost EXP Shares from trading, there should be a pop-up that says something like: This Pokemon has an item on it. Are you sure you want to trade? YES NO
When it gives you the option to trade and says do you want to trade you can check to see what item the Pokemon is holding
Like Voultronix has mentioned, when trading, you can easily hover over your Pokemon and look at what item it is holding. Whilst the idea could be useful in the event that you are unable to look at what item your Pokemon is holding, it is less useful here as you can easily check to see what is on your Pokemon.