Tick template

Changing the look of the tick marks can be done by setting the TickTemplate property, which is of type DataTemplate.

<telerik:RadSlider Maximum="10" TickFrequency="2" TickPlacement="BottomRight"> 
    <telerik:RadSlider.TickTemplate> 
        <DataTemplate> 
            <Grid> 
                <Ellipse Width="5" Height="5" Fill="Black" /> 
            </Grid> 
        </DataTemplate> 
    </telerik:RadSlider.TickTemplate> 
</telerik:RadSlider> 

If you want to display the numeric value of each tick, you have to add a TextBlock to the template and bind its Text property as shown in the example bellow:

<telerik:RadSlider Maximum="10" TickFrequency="1" TickPlacement="Both"> 
    <telerik:RadSlider.TickTemplate> 
        <DataTemplate> 
            <Grid> 
                <TextBlock Text="{Binding}" FontSize="11"/> 
            </Grid> 
        </DataTemplate> 
    </telerik:RadSlider.TickTemplate> 
</telerik:RadSlider> 

More information how to use a value converter and display not only numeric values, can be found here.