There are a few spell-based mods that do this.
Detect Item adds a spell that puts a particle effect on items, making them obvious on the ground or behind rocks. It's a "beta" quality release and it's been years since, so it's unlikely to get polished up, but it works. Requires OBSE v0017 or later.
Finders Keepers adds a whole lot of spells for detecting different objects by adding shaders/particle effects. It also has spells that can detect different NPC aspects, such as Vampirism, disposition, etc. Some can be used simultaneously, and some will cancel others—so you can, for example, have Detect Weapons and Detect Keys running simultaneously, but casting Detect Alignment will cancel any other Detect spell. Clearly it's not as convenient as a one-size-fits-all Detect Items spell, but this might feel more "realistic" for some players. As a bonus, though, many of the item-detection spells will reveal things inside chests as well as on the ground. Requires OBSE v0013a or later and the latest official patch.
Some of the big magic overhaul mods may also include item-detection spells, but I haven't found a comprehensive guide to their included spells quite yet.
In the following path, %STEAMROOT% refers to your Steam install location, eg. C:/Program Files/Steam, and %STEAMUSER% refers to the name that you use to log into Steam. (These aren't environment variables Steam sets, so you'll have to replace them yourself.)
The install location for Garry's Mod addons is %STEAMROOT%/steamapps/%STEAMUSER%/garrysmod/garrysmod/addons.
Best Answer
I would recommend several add-ons in addition to MageNuggets which Ward mentioned, both of which are updated regularly.
SpellStealer
From the Curse page:
You can download it here. This add-on is more focused on just SpellSteal prompts, whereas MageNuggets has many Mage specific functions.
Power Auras
This is personally my best recommendation, as Power Auras is good for many other things, so chances are you can leverage the functionality for many things, if you aren't already! Using PowerAuras, there is a "Stealable Spell" option that you can set for a given aura.
From the Curse page:
You can download it here.
There is an active wiki for Power Auras maintained here, and you can find many detailed tutorials on the web for Power Auras, but I would recommend this one, which is for a slightly older version but the basics still apply. It may look daunting with all those options at first but it can be very powerful, even more so than the new in-game "Spell Alerts", which were basically a Blizzard implemented copy of Power Auras due to the popularity of the add-on.