I have been given some yellow orange coloured beads that I was
told were South American topaz. I've not heard of this and am
suspicious that it is another term or name for a synthetic or
simulant. Any info appreciated.
Find someone with a refractometer who can take an R.I. reading on the
stuff for you. South America is a big place with lots of gem
locations. Topaz is certainly possible, but so is Citrine quartz,
which is often also (incorrectly) called topaz. And then, as you
say, there are simulants, synthetics, glass, etc. My bets would be
either actual topaz or citrine quartz, or glass, rather than more
sophisticated synthetics… But just from the info you gave, there’s
no way to know beyond stating it could be topaz, or not. Glass, of
course, is easier to identify than the actual gem materials.
Usually, you can do that with a loupe, or at most, a polariscope
(which you could make from an old pair of polaroid sunglasses, if you
don’t have one.)