How to know if a paper is allowed (by publisher/journal/conference) to be put on arXiv?
Check the publisher's website. They should have policies for putting papers on arXiv.
For example here is Elsevier's policy page, and:


Authors can share their preprint anywhere at any time.
If accepted for publication, we encourage authors to link from the preprint to their formal publication via its Digital Object Identifier (DOI). Millions of researchers have access to the formal publications on ScienceDirect, and so links will help your users to find, access, cite, and use the best available version.
Authors can update their preprints on arXiv or RePEc with their accepted manuscript .


So for this publisher the answer is &quot;yes&quot;, you are allowed to put papers on arXiv as well as update them once the peer review process is complete.