Generally... I can tell from the URL enough to suit my needs:

1) If its a URL in a SPAM... assume its a [:'(]porn[:'(] site and add it to the blocked list. (don't bother sending me spam... I just block the e-mail address... forever.)
2) If its SUPPOSED to be an AMA Site it should have www.modelaircraft.org/XXXXXXXXXXx form (well it used to be that way...[&:])
3) If it is a XXXXXXXXX.de/XXXXXx or XXXXXXXXX.du/XXXXXXx its on a server that supports trojans, viruses and other garbage... stay clear.