Can Welsh ancestry be determined by surnames?

[Dollander]

Well, ap or ab, depending on the name; specifically it seems ab was favored for names beginning with a vowel (e.g. ab Ifan --> Evans/Bevan).

Also keep in mind that there was no Richards clan. Plenty of unrelated people wound up with the same surname when Wales made the transition from the patronymic system to the modern concept of family names, simply because their fathers had the same given name. So you won't be stepping on any toes by wearing the Richards/Pritchard tartan...or Roberts/Probert, for that matter.

And before the conversation gets derailed, I'll remind everyone what Rocky and Eric said regarding the newness and "unofficial" status of the Welsh tartans in one of USA Kilts' recent Q&As: every tradition starts somewhere.