Op****tunity Alert

Hi

I want to create an alert on the Op****tunity form that reminds a user to 
associate an op****tunity with an account.

Using the Onload event I need to write jscript which checks to see if the 
Potential customer is associated with a Contact then alert the user.

I am unable to reference the lookup correctly. My jscript is:
if (crmForm.all.customerid.lookupItem[0].type = '2')
alert ("Please associate the Potential Customer with an Account")

Would someone please assist and help me to reference the lookup correctly?
I 
know that 2 = contact so I just the field type.

Many Thanks
Mark