What specific behaviour confirmed the existence of the W and Z bosons at the UA1 and UA2 experiments?
Since Lubos did not make an answer of his comment I would suggest reading the "discovery" paragraph in the Wiki article.
The "specific behaviour" are the characteristic decay modes of each particle, that clustered at the masses finally assigned, as the reference in the comment above shows.
You need a library to go to the original discovery references, but the decay modes are in the article in this link.
Edit: from the last link: