Просто установите его как проверено:
<body>
<form>
<input type="radio" name="amount" value="10"/> $10 
<input type="radio" name="amount" value="25"/> $25 
<input type="radio" name="amount" value="50" checked="checked" /> $50 
<input type="radio" name="amount" value="100"/> $100 
<input type="radio" name="amount" value="250"/> $250 
<input type="radio" name="amount" value="other"/> Other
</form>
</body>