How to check and get json object in WP database?

To check if a string from database is like a:2:{i:3;a:4:{s:5:"title";s:0:"";s:4:"text";s:503:"...}}, you can use the function is_serialized()

To get the data as php array :

if(is_serialized($data_as_string)){
    $data_as_array = unserialize( $data_as_string);
}