It is very easy to tell if your car has abs or not (aside from the fact that the breaking feel is different) by looking under the hood since, if your car has abs you will be able to see the abs unit. Its hard to miss since its a big unit with several hardlines going to it but this link has a picture of what it looks like
Identifying ABS - 240sxONE Forums. I think the picture is of a s14 but its the same in both chassis. If your car does not have that then you do not have abs. If I had to bet I would say that your car does not have abs since most 240s don't. The difference between the abs and non abs driveshafts is that the abs one is slightly shorter to make up for the longer abs differential. There is usually enough play to make a abs driveshaft fit in a non abs car and vise versa but you should still try to get the one that matches your car if you can. The fact that it is going on a Rb20 doesn't matter since the Rb20 uses the same transmission as the ka/sr.