Wire Disentanglement

hi all.

having looked at hundreds of puzzles, i've noticed tons of
similarities (of course). similar designs just remodelled and
presented in different shapes. my question is, how would one know if a
new design is possible to solve or not? i'll try to clarify my
thoughts... a certain site offers a puzzle as a novelty item. they
state that it is impossible to solve. when they created that puzzle,
how did they know it just wasn't really really hard?

are there certain things you can look for, or that must be present
when designing a new puzzle (wire disentanglement, specifically). as
an example, someone might say, "if you imagine re-twisting the
finished puzzle into a circle shape, the string will be on the outside
of the cirlce...so you know it's solvable."

that's just an example of course (and not necessarily true), but thats
kind of what i'm looking for. any help would be great.

thanks