Back in the day there were some mods that let you look at parts to get a general idea of how damaged they were. From the sounds of it, however, those mods no longer work, and in addition to not being customizable, they're also not open source and use Mod Loader Pro, which is now abandoned - meaning it was impossible to make an updated version. Finally, condition text didn't appear to update in realtime; only between saves, which means you could be looking at a broken piston and see that it was apparently fine!

Part Inspector is a mod that lets you look at car parts to see how damaged they are, written for modern versions of MSCLoader and the game. In addition to being written from scratch to be lightweight and clean, it features a lot of customizability; you can choose where the integrity of the part displays (either in the name itself or in the white interaction text) in addition to how it displays (exact percentage, general description, or just intact/broken). See the images for examples.

Built and tested on MSCLoader 1.2.12.

Notes

• Part Inspector is built for MSCLoader, not Mod Loader Pro. It might work on MLP, but I haven't tested and MLP is abandoned, so use it at your own risk!
• At the moment, only fully unbolted parts show their integrity - just think of it as loosening the part to get a good look at it. You can still keep it on the engine, you just have to loosen the bolts to the point where you can take it off if you wanted. If I can find a workaround for this, I'll use it!
• The mod includes logic for all twelve parts that wear down over time (pistons, fuel pump, water pump, alternator, starter, clutch disc, gearbox, crankshaft, head gasket, rocker shaft, spark plugs, and alternator belts), as well as simple inspection for the block and oilpan that show if they're broken and dirtiness inspection for oil filters that show how filthy they are.
• Safe both to add to and remove from an existing save; parts won't be affected.
  

You can find the source code for this mod on GitHub, under the GPL v3 license (see Permissions on this page for more info.) If you find any issues or bugs, you can report them there or in the comments here.

Settings

• Display location - Determines if text will display as part of the part's name, or in the center of the screen. Defaults to the part name.
• Display precision - Can show either exact percentage, a general description (i.e. mint condition, shoddy condition), or intact/broken. Defaults to exact percentage.
• Text update frequency - To prevent possible performance issues, each part's displayed text only updates every few seconds, since condition is unlikely to change rapidly between you actually checking the status. It defaults to 10 seconds, but you can crank it all the way down to 0 to make it happen in realtime.
• Verbose logging - This is a debug option that will put a bunch of extra info in your debug log. If you have any issues to report, turning this option on and providing the log with your report will help a lot in diagnosing the problem.
  

Known issues

• There's some minor text flickering when using the "interaction text" option. Nothing too big of a deal, but it's there.
• When looking away from a part while using the "part name" option, you can see the unmodified name for an instant after. Also not a big deal, but it's there!
• When buying new parts from Fleetari, it might take a little bit for the text to update with the new durability. This is because the game doesn't seem to technically make a new part - it just "repairs" the existing one and moves it to the table.
  

Condition thresholds
Regular parts:

• Mint - 90% and above
• Great - 90% to 80%
• Good - 80% to 65%
• Decent - 65% to 50%
• Shoddy - 50% to 35%
• Poor - 35% to 20%
• Bad - 20% to 10%
• Terrible - 10% and below
  

Spark plugs function normally even at lower condition, and have different thresholds to reflect this. Alternator belts use these too, due to code similarities:

• Mint - 90% and above
• Great - 90% to 65%
• Decent - 65% to 25%
• Bad - 25% to 15%
• Terrible - 15% and below
  

Oil filters track dirtiness as an increasing value, rather than a decreasing 'integrity' value like other parts:

• Brand New - 0% to 20%
• Clean - 20% to 40%
• Grimy - 40% to 60%
• Dirty - 60% to 80%
• Filthy - 80% and above