How to create an Access query to find # symbol into a field?
For Access you can use:

WHERE obs LIKE '*[#]*'


There are three wildcard characters in Access that can be used with LIKE operator:

`*` for a string of arbitrary length (equivalent ANSI: `%`)
`?` for a single character (equivalent ANSI: `_`)
`#` for a single numeric digit 
`[` for escaping


If you want to search for one of those wildcards, you have to enclose them in brackets: [ ]

You can find more info in the msdn article: Separate But Equal Wildcard Characters in SQL Server and Access