You can tell by going in the config pages of the DSL modem (not your router) See following:
You can read about Interleave vs Fastpath here:
If you are on Interleave and want to go to Fastpath, then you need to contact higher level techs at ATT. Use the ATT direct forum at dslreports.com
Been playing around with different settings and nothing has helped. So called ATT tech support and talked with someone in CA. who told me the modem was configured in fastpath mode, and that everything was performing as it should. Did not try another modem as I don't have access to another one without buying one, but the tec said that they have had a few problems with that model # (Speedstream 4100) which might be why I get high latency on Yahoo.com and normal on other sites, using pingplotter. Any other thoughts? otherwise I will live with it for now and try to find another modem to try. Thanks much for your suggestions.