Newbie question regarding single table inheritance

In STI, how does one tell whether a subclass uses a certain attribute or not?

Eric

"When defining the table, remember that the attributes of subclasses
will be present only in the table rows corresponding to those
subclasses"

Yes, but I think if I were to set attributes corresponding to a
sibling class, Rails would still let me, assuming I haven't put in
place any validation that the field cannot be set.

So, it seems unless I validate that only the fields for a subclass can
be set, the only way to tell is to ask the original developer or
figure out based on intuition or reading the code.