Netherlands 2 1/2 Gulden 1858 - Genuine ?

It looks like it might be genuine to me. I don't see anything worrying at first glance (except that the coin is a bit off center on the obverse: see the edge at 4 o'clock). 1858 was the most common year by one (1872) for number of coins struck. Except from a few dents on the edge, this coin seems to be in very nice condition. I'm a bit envious as my sample of this year isn't that nice...

I'd advise you to measure it and weigh it. It should be 25 gram and have a diameter of 38 mm. Also check if the edge has the inscription GOD * ZY * MET * ONS * ('God be with us').

Let me put it this way: if this is a fake, then I think our Chinese 'friends' have raised their hobby to a new level again...

Agreed, looks genuine from the images. Good details, proper denticles, inscription looks in line and not all over the place, only shame is that its got some dents, but that's understandable. Edge pics + weight would be the last bits needed for authentication as mentioned.

Commenting about "raising the bar":
I've come across a good number of fakes now that really look so good, I doubt if anyone could find fault with them in a posted picture alone.
I've even had experienced collectors doubt or question my claim of being fake looking at the coin in hand.
Then they learn they are fakes after other attributes are examined.
But yet we do get a lot of such posted requests here.
It seems to me these fakes coming out of China are made by a variety of sources.
And they also seem to be at different levels of quality.
They range from the very poorest and most obvious to good, better and best.
For that reason I don't like to say if a coin is genuine or not unless there are obvious flaws that I recognize

Tiny drop of household bleach.