I have a long form ~22 fields with no client side validation, and it gives an error message which just reads "Error", after i hit the submit button? how can i go about testing it? any suggestions.
Report that it is unusable as user is not given relevant information. This is a serious bug, especially if it is an important form.
If you need to say something more in it's current state, ask for specification on what should be valid values or value combinations for each field. Then test border cases of each field separately and maybe some random other combinations for a good measure.
If the application under test is a web based (unfortunatelly the question is not exact enough), you can get detailed error message from the browsers. Firefox or Chrome can give you a lot info trough developer tools.
In Chrome: Press Ctrl+Shif+I or F12, from Context more info here: https://developer.chrome.com/devtools
In Firefox: Press F12 or use Firebug Plugin, from context Inspect Element more info here: https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector
Interresting parts to get more details about the error are:
- Network folder - each request is here with its response
- Cookies folder - sometimes cookie contains important information about transaction/s