Do research online
Easiest way to spot fake antique is to do research online, use google camera to find similiar items,
if it's for example alleged "French Saber 1895 model", compare it to authentic ones online.
Note that some antique may lack few parts due long age, it's not a reason to worry
Look for simplicity and unnatural looking blade
Many modern replicas has way simplier parts.
cheap replicas lacks the details of original model, athough it's not unequivocal that it means your sword is 100% fake.
Look for luxury paterns is sabers for high-ranked persons. Sabers made for soliders aren't often too detailed.
Look for imperfects
Saber made 500 years old are almost impossible to be fabric-new, look for chips, natural looking patina and missing parts of the item.
Items commonly used are most likely to look used, rust and natural pittings are often helpfull in indefiting sabers.
Blade itself
As i mentioned earlier, imperfects in the blade might be helpfull. There are many cases that replicas make perfect blades, sometimes it doesn't even match the original in cheap copies. If blade is shiny but looks used, it might indicate that item is not fake but ceremonial or cultural.
Many Bayonets and sabers was made for parades, these are commonnly i pretty good shape due to lack of usage during years. These weren't designed for combat but for demonstration of power during marchs and events.
Don't forget to look at bottom position of saber, after blades were mounted to handles, creators had to hide or hammer out the start of the blade at the end. If it's a black dot is there inside metal stain, it might indicate the blade was refitted.
Decorative or combat sword?
We already mentioned combat sword and parade swords,don't look for combat sword characteristic in old decorative sword.
Often people made decorative spades, to decorates their households. There could be antique decorative swords!