You can edit the Style of RadSlider and set a tooltip that is bound to the Value property:
I've attached a project containing a sample solution to this scenario. Have a look at it and let me know if it helps. One further clarification, since the XAML of RadSlider is quite large, you can delete the unnecessary stuff. For example, if your slider is horizontal with just 1 thumb, you can remove HorizontalRangeTemplate, VerticalTemplate etc.
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