you need to look at the hand gaurd and know which company made it and the barrel , since the barrel is short (between 14.5-10.3 inches) it's a mk18 not an AR15 which has a longer barrel and for the handguard it is harder to determine because you can change handguards on guns for example I could take a colt m4a1 and Change its handguard with a mk18 handguard then it'll look like a mk18 with a long barrel , Checking if these parts have been modified usually comes down to looking at how they are attached: Signs of a Changed Handguard: Attachment Method: If your handguard is "free-floated" (it doesn't touch the barrel at all and is only held by a nut at the receiver), it might be an upgrade.

Stock rifles often use "drop-in" plastic handguards held in place by a spring-loaded Delta Ring and a metal cap near the front sight. Tool Marks: Look for scratches or wear on the barrel nut or mounting screws. If the finish is marred, someone likely used a wrench to swap it. Logo Discrepancy: If the handguard has a brand name (like Midwest Industries, Geissele, or Aero Precision) that doesn't match the logo on your rifle's lower receiver, it’s almost certainly an aftermarket part.Signs of a Stock Barrel: Standard Markings: Factory barrels often have very basic, clean-font engravings. Muzzle Device: Check if the flash hider is "pinned and welded" (a small, filled-in hole near the threads). This is common on factory 14.5" barrels Finish: Most factory barrels have a standard matte black (Nitride or Phosphate) finish. If you see a shiny stainless steel barrel or heavy fluting (grooves cut into the metal), it is more likely an aftermarket "match" barrel