full-duplex speakerphones?

How can you tell whether the speakerphone on a particular model works
at full-duplex? I think I've read posts here and there on the issue but
neither Sprint nor the manufacturer's websites make this distinction. I
just wondered if one has to rely on anecdotal evidence when choosing a
phone.

I've read reviews on many Sprint phones and will probably end up
getting one of the Sanyos. They seem to have the best reception from
general user comments. I currently have the RL-7300 but believe the
speakerphone runs at half-duplex, making conversations quite
frustrating at times.

C.H.