To validate at server server side, there are 2 ways,
Either use validation control or validate manually.
1. To apply validattion using validation control you can use CustomValidator. Using this control you can apply client side validation as well.
Check the following link to get the idea of all the validation control,
http://www.w3schools.com/aspnet/aspnet_refvalidationcontrols.asp[
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2. To Validate manually, just check your control value from code at server side and show message on label if the data is invalid.