How to set an alert when input is not numeric

[Greg Walker]

I have a simple form I created to calculate gross pay after input of pay rate and hours worked minus deductions. I need some sort of function to alert when non-numerical is input in both of these fields, which is the snippet of code below. I am very new to this and so far I am happy with my results, just trying to dummy proof my form.
Thank You!

<font face="arial, helvetica" size=4>Please enter both the Hourly Rate of Pay (Pay Rate) and Total Hours Worked (Hours) in positive numbers
</P>
<br>
<center>
<form name=paycheck>
<table border=9 cellpadding=3 cellspacing=2 width=15%>
<tr>
<td colspan=1   </td>
<td bgcolor="#c0c0c0" align=right><b>Pay Rate:</b></td>
<td><input type=text name=Hr_Rate  size=5 ;"></td>
<tr/>
<tr>
<td colspan=1  </td>
<td bgcolor="#c0c0c0" align=right><b>Hours:</b></td>
<td><input type=text name=Hr_Worked  size=5 ;"></td>
</tr>

[Dormilich]

try to read the value by the isNaN() function, if it’s not a number it returns true.