Ok, I found it myself. I'm posting this for people facing similar problem.
To ask SOLR how is indexing going try this url:
/solr/admin/cores?action=SUMMARY&wt=xml
Most interesting values here are:
<long name="Approx transactions remaining">0</long>
<str name="Approx transaction indexing time remaining">0 Seconds</str>
which tell how much work SOLR thinks it has to do.
To get more information about the status hit:
/solr/admin/cores?action=REPORT&wt=xml
to get <long name="Index error count">
to tell how much documents are not searchable. You can also get what caused this hitting
solr/afts?q=ID:ERROR-*&wt=xml
also if you have more than 50 problematic documents add &rows=
providing number of problematic indexes you want to retrieve.
Note: If it's Alfresco's SOLR then the link goes: solr/alfresco/afts?q=ID:ERROR-*&wt=xml