It's a black art

experience is the best guide, but some tell-tale features:
The obvious ones:
1. the plot summary or credits on the back of the case are for a completely different movie! (happens a lot with bootlegs that appear whilst the films are in cinemas, sure to be bad quality)
2. If the printing on the sleeve or disc do not have that "crisp" professional quality, i.e. they look like they've printed on somebody's home printer.
The ones that require some research:
1. Artwork/packaging is different from known legit releases - e.g. does not come with a slipcase when it should (note that having a slipcase or the right artwork doesn't guarantee it's NOT a bootleg)
2. Disc is advertised as "Region 0/Free" when the official release was region coded... but this is difficult because many legit Asian dvds claim to be region coded on the box but don't bother coding the actual disc!
3. Disc contains subtitles/audio tracks that are not on the legit releases - e.g. if you see anime on ebay with Chinese + English subtitles, 90% certain bootleg.
4. Selling new products way under the retail price... if you see somebody selling new/sealed Japanese DVDs for $14.99, they're bootlegs.
With ebay you can often ID sellers who are carrying bootlegs by looking for one or more items that match the above - e.g. at the moment there is no legit Tom Yum Goong DVD, so searching "Tom Yum Goong DVD" on ebay will find you several bootleggers straight off. If they're selling one or two bootleg titles, chances are that everything they sell is bootlegged.