It's tough to say for sure, given that you don't know for sure what kind of glue was used.

If you think it was CA glue (superglue), you could try a cotton swab dipped in acetone, which will dissolve the glue.  You have to be careful, because it can also melt styrene, though a swab soaked in the solvent won't be hard to control.  I have used this method myself.

If it looks like the glue melted the plastic as part of the bond, I'd say that it was a glue designed for use with styrene, which melts the plastic to create a weld.  That kind of glue, you cannot remove.  You'll have to sand down the former join to get a clean surface.

You might be able to scrape the remaining glue away, too, with an X-Acto knife.

That's about all I can think of, without seeing the piece or knowing more about the glue that was used.