As a collector of coins, I guess anything can be faked if there is enough profit in doing so.
So you have to ask yourself, what does it take to make a passable fake FF insert?
You'd had to create printing pads with the correct fonts and you have to construct them the same as the genuine FF inserts. (this can get costly taking into account the different iterations printing and pad wear that evident on gen FF inserts)
So in order to make sense in terms of profit.. meaning if there was an operation faking them, they would need to flood the market to make a profit.
Do you see a flood of Fat Font Inserts into the market? Nope. Actually it's the opposite, there is more demand than supply and it's been that way for the past 5 years or so.
And the ones that are coming on the market are all different, in terms of age, patina, fade, wear, mk0, mk2, mk3 etc. No 2 inserts are rarely the same.
You don't see for example 500 mint condition long 5 kissing 4 inserts hitting the market.
The only known fake is the Yuki FF Insert and genuine fat font inserts have some "printing" tells vs Yuki.
The silver font printing on gen FF inserts are printed on top of the black. On the Yuki, the silver printing is exposed metal.
You can tell this by trying to fade the yuki with bleach, the silver fonts won't fade out as the black fades out.