Heads on a specific branch

[Paul Sargent]

So I've just had a minor nightmare where I told someone the reason he
couldn't push was because he had multiple heads on the branch, and he
needed to merge them. He went away and merged all the heads which
included several named branches, and then proceeded to push once he
got down to a single head.

I've now got someone else asking me a similar question. Is there a
command I can give them that will identify the heads that are in a
specific branch (or the default branch)?

Ideally it would be a command that worked on hg ~1.5.

Thanks