How can I check in my bashrc if an alias was already set

If you just want to make sure that the alias doesn't exist, just unalias it and redirect its error to /dev/null like this:

unalias foo 2>/dev/null

You can check if an alias is set with something like this:

alias foo >/dev/null 2>&1 && echo "foo is set as an alias" || echo "foo is not an alias"

As stated in the manpage:

For each name in the argument list for which no  value  is  sup-
plied,  the  name  and  value  of  the  alias is printed.  Alias
returns true unless a name is given for which no alias has  been
defined.

Just use the command alias like

alias | grep my_previous_alias

Note that you can actually use unalias, so you could do something like

[ `alias | grep my_previous_alias | wc -l` != 0 ] && unalias my_previous_alias

That will remove the alias if it was set.