Verify My Database Isn't Slammed

I've got a dedicated database server running PostgreSQL 9.1.2 which I
thought was working fine but I am now having users slowly trickle back
into the office and using some PHP web applications that use my
database server and strangely the site will load forever for some
users. They simply close the browser and or wait a few minutes and
then they can get in fine. I checked my Apache web logs and found this
error:

[Tue Jan 03 11:34:09 2012] [error] [client 192.168.1.16] MRBS:
Fatalerror: unfortunately the database is not available at the
moment.,referer: https://mysite.tld

I have also a webmail application called RoundCube that over 200 users
login to and this connects directly to the database server and there's
never been any communication issue what so ever. I'm going to assume
the issue is a configuration setting in MRBS in speaking with the
database and not the database itself but how does one who is extremely
novice with PostgreSQL determine if their database server is over
worked or running fine?