Just looking at the rules for Decieve and just how far actually counts as decieve?

say for example you passed on your lie to convice someone they were a potted plant or hedgehog would that still count?

according to the book a lie which contradicts what a person can see is a -30 test so in that case if i had Decieve +30 on my sheet id be making a straight check and seeing as most characters dont have scrutiny which is used for telling if someone is telling a lie this would seem to be absurdly ridiculous, so what would you say is actually allowable?

It's lying, so anything a really good liar could convince you was true. Now, I doubt even the greatest conman in the world could convince you you're a potted plant. Just use common sense, really. Possibly eering on the side of yes, in case of gray zones and really good Apostates.

Like Reverend Mort said, it's mostly a matter of both skill and common sense.

My GM used to have a really bad issue with this, so long as you rolled well your roll could accomplish anything. Roll a 09 on a decieve test? He is a hedgehog. I brought it up to him and explained that the whole point of having a GM was not only to craft the surrounding story, but moderate the capabilities of the players. If you could just go around convincing everyone they were your pet hedgehog then there'd be no difficulty to the game. Some things just cannot be tricked like that. A necron will never think it's an Eldar for example.

What you COULD do to facilitate that kind of Deceive test being legitimate is use hallucinogens. Then it's easy peasy. 

hmmm well my character in the game im currently in does have a full back pack of hallucinagen grenades might have to start using those thanks for the help guys.