Check if a string contains only number

TRY_PARSE will let you compare the input value to any/all numeric datatypes you decide to allow -- for example:

SELECT 
   TRY_PARSE('123.456' as int) as [int],
   TRY_PARSE('123.0' as float) as [float],
   TRY_PARSE('d123.456' as int) as [int],
   TRY_PARSE('d123.456' as float) as [float]

FWIW -- ISNUMERIC is often suggested, and is certainly the best-sounding function name :-) -- but doesn't work the way most folks seem to expect. (It allows math and currency symbols, etc.)

I would suggest try_convert():

select (case when try_convert(col, float) is not null then 'valid' else 'invalid' end)

The one possible downside is exponential format; 1e6 is a valid number for instance.

An alternative is the where approach; you just need more complete logic:

select (case when col like '%[^0-9.]%' then 'invalid'
             when col like '%.%.%' then 'invalid'
             else 'valid'
        end)

There's a sql server built in function:

Select CASE WHEN isnumeric([fieldname]) THEN 'Valid' ELSE 'Invalid" END