built or not

i need to know if a structure is built or not.
I see only this way :
add in a group the unit begins to build and remove when she cancel / finish the built / die / is removed.
So if the unit is in the group, she isn't built

a better idea ?

When a structure has finished construction, give it custom value
Then just use an integer comparison to check Unit Custom value.

i already use custom values for something else.
i can use game cache too, and i think i will use it so.
that's true that is easier to check only the finish event, thx

EDIT : Hmm if i check only the finish event it will leak with the game cache ...