I got this info from the trivet collectors board on ebay...
Quick Ways to Differentiate a Repro from a Genuine Antique:
1. Check the reverse. If the name of the manufacturer appears the trivet is a reproduction. For example: John Wright, JZH, Wilton, Emig, Dalecraft, Virginia Metalcrafters ... all mid 2oth Century trivets. (note from Charlee: I would also add Griswold, Wagner, and Lodge to this list)
If there are only letters or numbers on the reverse, it could indicate either a mid 20th Century trivet or an older, vintage casting. In the book Trivets & Stands Ellwood states that Light Egyptian letters/numbers were occasionally used between the late nineteenth century and WWII.
Light Egyptian Font
2. Rubber caps on the feet? REPRO! These caps were first seen on trivets in the 1950s.
3. Heavily reproduced design? BEWARE! It is most probably a repro, either 1950s or earlier.
4. If the leg length is >1.25" it's most probably an older or antique trivet. The 1950s repros were made with shorter legs for wall and tabletop display. Older trivets had higher legs to protect the table surface from the heat of either the sad iron or hot pot.
I also look for evidence of grinding wheel marks along the edges where the gate marks should be. These will be fine vertical lines at fairly even spacings. Grinding wheels weren't introduced until after 1895 according to Trivets & Stands.
And if the grinding marks are irregular, then they were from a hand file, predating 1895
You have to learn to recognize the metal. The old Iron has a different look because of its metals content. (Nuther Charlee note: Old iron will rust "brown" (think rusty old bridge) while newer iron will have the more orange rust.)
(Yet another "Charlee note") British trivets can be dated by the RD number on the back. I have a Carron turtle trivet that I can tell you the DAY it was made, because of the RD diamond on the back....