The easiest way is to google a couple and then look for the ID in the results. The entries from genecards are often useful for this, since they draw from a number of different databases. In your case, those are Entrez IDs.
The PSI-MI (HUPO Protein Interaction Standard Initiative - Molecular Interaction) consortium has developed an ontology to describe protein interaction. This ontology contains a branch for database references. Most database reference are associated with a regular expression (id-validation-regexp).
DAVID has a tool to convert gene ID's to those of a particular data base. For the input Identifier you can select "not sure" and it will make its best guess. I've had some success using this tool, so it's worth a try if googling doesn't give you quick results.
edit: it looks like the DAVID database isn't updated, so it may not be too helpful for you.