Welcome, Monsieur Silk! We hope that you came by this item legitimately, and didn't find it in one of your wallet collection... ;-)
More seriously, it's always going to be very tricky to answer a question like this.
You've not provided any context for how you got the piece, so we don't know what checks you did before buying it: did the seller give you any background as to where, how and why it was obtained originally? Those details are really important, and help to establish the chain from then until now.
Was there any proof which established the history as plausible (for example, could they show you a photograph of the occasion on which it was given)?
Did the seller try to disassociate themselves from authenticity? You see this a lot on places like eBay, where a seller - after paragraphs of how wonderful their "Picasso"/ "Rembrandt"/ "Jack the Ripper" autographed sketch or letter done in magic marker on a sheet of photocopier paper is, then says "I cannot guarantee that the item is genuine, and don't do refunds". It should ring alarm bells.
Was the price too good to be true? While it is always possible to stumble upon a signed copy of some rare book in a dusty second-hand shop, or find a bargain Van Dyck when someone doesn't know what they are selling
, it's always sensible to try and think if the asking price and the item seem to agree.
Hergé autograph sketches are sought after and desirable to collectors, and thus have a monetary value; but, in practical terms, they are also quite simple to reproduce, being effectively pen on paper, which is easily obtainable.
If you can fill in any of the above, then you will start to build up a better picture for establishing authenticity.
You can, if you want, go through the Tintin.com website to the expert panel which examines items to make a declaration of authenticity; it will still only be an opinion (short of them being able to show fraud - e.g. if it's Hergé's signature on a 1984 calendar...!), but it will be a good opinion.
Without any more to go on than the photo, I have to say that I'm not entirely confident about it - the Snowy looks okay, but the Tintin is a bit heavy, the signature looks a bit angular, and the writing doesn't look fluid and neat enough. I'm also not sure about the paper - it's unusual to see what appear to be rough edges, I think.
However, as I say, that's purely based on the picture, and I've not got any context: was he old, was he ill, was he writing it on a napkin on the side of a suitcase at an airport...? Had he signed a thousand autographs at a book-signing, and was tired? It's just not possible to say definitively.
So please let us know if you have any other corroboration from the seller, and a history of the piece; then things might get easier!