I don't think there's any way to tell on a 70, unless you order a Marti report. http://www.martiauto.com/reports2.cfm
On the 70's (if it has not been re painted) it will have Grabber stripes on the side and the hood without the word GRABBER in the side stripe. It will have a rear spoiler but a flat hood, Blacked out rear panel and a black grill. Other than that you will have to get the report. Door panels are normal, no Sport Lamps. I do not believe they made an early 70 (69.5 Key in the package tray) Grabber. At least I have never encountered one. and it has the normal 91 in the VIN. This is all that I can tell from my 70 Grabber. There may be a few more things but I have not looked at mine for quite some time.