The unique identifier datatype is used to uniquely identify a row across various databases in the network. The value has 36 characters inclusive of digits from 0 to 9,alphabets from A to F and hypen. Suppose you have a string of 36 characters and want to know if it is really an uniqueidentifier. You can use either of these methods

Method 1

declare @uid char(36)
set @uid='6F9619FF-8B80-D011-B42D-00C04FC964FF' 
select 
	case when @uid like 
	'[0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F]-[0-9A-F][0-9A-F][0-9A-F][0-9A-F]-[0-9A-F][0-9A-F][0-9A-F][0-9A-F]-[0-9A-F][0-9A-F][0-9A-F][0-9A-F]-[0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F][0-9A-F]' 
	then 
		'yes' 
	else 
		'no' 
	end
	

Method 2

declare @uid char(36)
set @uid='6F9619FF-8B80-D011-B42D-00C04FC964FF' 
select 
	case when @uid like 
		replicate('[0-9A-F]',8)+'-'+
		replicate('[0-9A-F]',4)+'-'+
		replicate('[0-9A-F]',4)+'-'+
		replicate('[0-9A-F]',4)+'-'+
		replicate('[0-9A-F]',12) 
	then 
		'yes' 
	else 
		'no' 
	end