You could do the following
In Asp.net, after checking the condition you could create a small javascript function with alert('msg') and using RegisterClientScript you could put it on the page.. In the html page load event (html event) call this javascript function you created from Asp.net. If the function existed it will show the message otherwise it will not do anyting..
Refer this codeproject url for how to use RegisterClientScript
Use & Call RegisterStartUpScript, RegisterClientScript and Client-Side Script[
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