Brass and primers are the first tells, plus the bullets.
Factory primers are generally seated consistently and precisely with no distortions or marks. Many reloads will show varying seating pressure causing noticeable flattening and/or marks on the primer metal from the seating tool.
Cases:
--mixed headstamps
--headstamps uneven or flattened looking
--very highly polished (jewel-like appearance) indicates probable reloading with tumble-polishing beyond what would be seen on factory loads
--visible longitudinal lines or striations on the case walls; sometimes left by resizing dies
--noticeable bulge forward of the case head; this area is seldom contained in the resizing die and frequently noticeable on reloaded ammo
--noticeable bulging at the base of the seated bullet; many sizing dies reduce case necks to minimum which results in some visible bulging when a bullet is forced into seated position
--wasp-waste appearance of the case; many semi-auto cases (9mm, 40, 45, others) have a tapered case profile, but carbide sizer dies reduce the diameter equally from case mouth to web, then seating the bullet forces the case mouth to widen significantly during bullet seating.
Bullets:
--visible ring marking on the nose or along the tapered forward portion; caused by imprecisely shaped seating stem during the force of seating
--jacketed soft point or jacketed hollow point bullets may show some displacement of exposed lead at the nose; caused by imprecisely shaped seating stem during seating
--cast bullets will show a mold line over the nose and down both sides; very few (if any) factory ammo uses case bullets.
I've been reloading for 52 years and my own show some or all of these tells.