A simple function that checks if a passed string is a GUID.

create function IsGuid ( @testString varchar(38))
returns int
as
begin
    declare @ret int
    select  @ret = 0,
            @testString = replace(replace(@testString, '{', ''), '}', '')
    if len(isnull(@testString, '')) = 36 and
       @testString NOT LIKE '%[^0-9A-Fa-f-]%' and
       -- check for proper positions of hyphens (-)  
       charindex('-', @testString) = 9 and 
       charindex('-', @testString, 10) = 14 and 
       charindex('-', @testString, 15) = 19 and 
       charindex('-', @testString, 20) = 24 and 
       charindex('-', @testString, 25) = 0
          set @ret = 1
    
    return @ret
end
go
select dbo.IsGuid ('DAB1df7F-BCB8-4935-9364-f140F64EE5F7')
select dbo.IsGuid ('{DAB1df7F-BCB8-4935-9364-f140F64EE5F7}')
go
drop function dbo.IsGuid