How can I see what processes are running?

From the ps man page:

-e Select all processes. Identical to -A.

Thus, ps -e will display all of the processes. The common options for "give me everything" are ps -ely or ps aux, the latter is the BSD-style. Often, people then pipe this output to grep to search for a process, as in xenoterracide's answer. In order to avoid also seeing grep itself in the output, you will often see something like:

 ps -ef | grep [f]oo

where foo is the process name you are looking for.

However, if you are looking for a particular process, I recommend using the pgrep command if it is available. I believe it is available on Ubuntu Server. Using pgrep means you avoid the race condition mentioned above. It also provides some other features that would require increasingly complicated grep trickery to replicate. The syntax is simple:

pgrep foo

where foo is the process for which you are looking. By default, it will simply output the Process ID (PID) of the process, if it finds one. See man pgrep for other output options. I found the following page very helpful:

http://mywiki.wooledge.org/ProcessManagement

have you tried ps aux | grep postgres? it really should show up if postgres is running. If it doesn't... how do you know postgres is running?

(note: it's a common misconception that's it's ps -aux but that's not correct)