How to verify if if checkbox is checked or unchecked in C#

How to verify if checkbox is checked or unchecked in C# ? Because method "element.Selected" does not work in this case.

Here it is html code if checkbox is checked:

<td class="dxgvCommandColumn_MetropolisBlue dxgv" onclick="aspxGVScheduleCommand('messageGrid',['Select',1],1)" align="center">
<span id="messageGrid_DXSelBtn1_D" class="dxICheckBox_MetropolisBlue dxichSys dxWeb_edtCheckBoxChecked_MetropolisBlue">
<input id="messageGrid_DXSelBtn1" value="U" readonly="readonly" style="border-width:0;width:0;height:0;padding:0;margin:0;position:relative;background-color:transparent;display:block;" type="text"/>
</span>
</td>

Here it is html code if checkbox is unchecked:

<td class="dxgvCommandColumn_MetropolisBlue dxgv" onclick="aspxGVScheduleCommand('messageGrid',['Select',1],1)" align="center">
<span id="messageGrid_DXSelBtn1_D" class="dxWeb_edtCheckBoxUnchecked_MetropolisBlue dxICheckBox_MetropolisBlue dxichSys">
<input id="messageGrid_DXSelBtn1" value="U" readonly="readonly" style="border-width:0;width:0;height:0;padding:0;margin:0;position:relative;background-color:transparent;display:block;" type="text"/>
</span>
</td>

Image: