Generally if you "hover" over the link in the e-mail, it will shows the true link, and when it is different from the one they put in the e-mail, it is spam.
If it asks for any personal info, or passwords, etc, it is spam. Generally, Paypal, your ISP, Ebay, and others sites that have spam sent pretending to be from them, have ways to forward the e-mail to them, so that they can deal with them. Often they are abuse@... fraud@...or some such e-mail address.