How to tell if package is installed?

[osx]

What's the best way to tell if a package is already installed? I can't seem to find a way to do it with apt-get.

Thanks.

[whoop]

I have mutt installed. If I type: "sudo aptitude search mutt" it gives me (amongst other things):

i   mutt      - text-based mailreader supporting MIME, GPG, PGP and

The "i" means it is installed...

[startrekker]

dpkg -l | grep packagename

[osx]

I have mutt installed. If I type: "sudo aptitude search mutt" it gives me (amongst other things):

i   mutt      - text-based mailreader supporting MIME, GPG, PGP and

The "i" means it is installed...

Thanks, that's what I am looking for.

Thanks also, startrekker, your suggestion works well too.

[dudumomo]

I usually use apt.
apt-cache policy mypackage

[osx]

I usually use apt.
apt-cache policy mypackage

Thanks. I like the output of this one, too.