You are correct that this is automatic: See "Advanced burst transfer is backwards compatible with devices that do not support this data type: if an advanced burst transfer is attempted with a device without this capability, the transfer will be downgraded to a normal burst transfer provided that a key property is not marked as required. This includes all advanced burst configuration options, including retry count extension and stall count." from section 9.5.2.27 of the ANT Message Protocol and Usage document.
"How can I tell if my slave device supports advanced bursting in the right way?" If you are referring to an ANT slave where the device you are writing SW for is an ANT master, then you do not need to know. You just try as advanced with the properties not marked as required and it will fall back on normal burst if not supported by the ANT slave connected.
You may also wish to read the ANT AN Burst Transfers document if you have not already. In particular section 8.
