This is a very informative site with casting numbers, etc.
Ok, so the 360 LA and 318/360 Magnums all have port sizes of roughly just over 2 inches tall by a smidge over 1 inch wide. The 318LA has ports that are almost 1/4 inch shorter and narrower. Also, the LA intakes bolt into the heads at a 45 degree angle, while the Magnum intake bolt into the heads at 90 degree, straight up and down angle. (That's why there are some considerations to remember when swapping between the two types). Also, the LA heads have a crescent moon in the center of the heads with an additional oval shaped port within in for the exhaust crossover. On a Magnum, it's solid metal. Also, Magnums have a 10-bolt valve cover, LA's have a 5-bolt cover. LA's have multiple V-belt accessory drives, Magnums have a single serpentine drive. Look for the sticker on the hood or the radiator support. It should list the engine size. The other way would be to get dirty. The engine size is cast into the driver side of the block. Typically the Magnums will have metal heat shields around the spark plugs. Look at your exhaust manifolds. That's LA. Magnums look graceful by comparison. The differences are obvious. If the valve covers are off, the LA has rockers on shafts, the Magnums do not. The rockers are on individual pedestals. That outta be enough visual cues.