Replicas of this coin are very common, they're sold by the dozen in tourist markets in Laos and Cambodia for the equivalent of 2-3 USD. This price immediately betrays the fact that they are fake - a chunk of silver this size would be worth 10 times as much (sellers will insist that they are authentic, though).
Judging by the color, the matt look, the peculiar staining, and especially the flat, unlively details of the coin, I would say that these coins are replicas and not made of silver.
On these very tourist markets, smaller denominations and non-precious metal coins are for sale as well. judging from the looks, the grades and the distribution of coin types (e.g. the absence from offer of Indochina 50 cts, which is indeed rare), I found no reason to believe that these were fakes as well (or am I naive?). Also, it was much harder to bargain down the prices of those items. But anyhow, beware, when it comes to Asian 'big silver coins' which have been obtained easily or bought for a bargain - these are very likely to be fakes/replicas.
I'm only an amateur, so please do not take my judgement for granted. In this case, I think it could be a real one, although the details of the sitting Liberty do confuse me a bit. It looks as if the wear was already on the dies when the coin was struck, and not as something that happened afterwards. Normal wear erases the higher parts of the details first, while wear on the dies expresses itself in shallowness of the deeper parts of the details and an overall flat and unexpressive image. 'Fake' makers have the habit to continue to use their dies for more coins, while official mints regularly replace them by new ones to maintain quality.
Look up other images of the same coin on internet and try to get a feel of it.
As pointed out earlier in the post, first establish whether the coins are actually made of silver. As a start, you can check the weight on an acurate scale (like jewelers and pharmacists have). The weight is actually written on the coins in this case (after the word 'poids' on the reverse).
