I'm looking for a script to run off a cron job or just manually that will take in a URL and tell me if the site is up... I'm not sure which function I'm looking for, should I be checking if a file exists, checking the return headers, etc... any help would be appreciated!

ping -c1 -w1 192.168.0.1 >&- 2>&- ||echo "unreachable"

If the return if ping is "false" (ie: an error occured), this command will ouptut "unreachable".
Adapt it to your needs.

That seems to show it's always down on most sites I tested...

I won't ask you if you have changed the IP adress...
Otherwise, it might be because either your server or the target sites are blocking ping requets (ICMP packets).

In that case, you need to use another tactic. But I have no solution right now in my hat...

Yea, I did change the IP...

I somehow did manage to get a cron job stuck which now emails me every minute that my website is down... is it spam when its sent to yourself?