Some general advice: If the first letter looks strange or shaky and if a letter is not written in one go - be careful!
How does someone sign? Is the person known for just signing with their name or do they add something (e.g. the date thing mentioned by Lucy). Another thing to check is if there are ink dots indicating that the pen rested in a particular place. If these dots appear in odd places you should be suspicious. Most people don't stop several times while writing their name. But this is hardly recognizable from a photo on the internet. That's why it's always risky to buy from eBay as you can hardly judge from a photo if a signature is authentic or not (unless it's very different from what the signature usually looks like or you are an autograph expert).
Better spend your money on one of those expensive M&Gs and let Meat sign an item of your choice. That way, you now it's really Meat's autograph and you have a nice reminder of a special event.
