Rose Business Solutions has a nice tip on how to warn a user that they are entering a test company. Its a hidden GP feature. You simply add to the end of the company name in company setup. Full info with screen shots on their blog.
Update: Cindy Mikeworth of Tas-In was kind enough to point out that another benefit of this is that active employees in the test company don’t count against your licensed employees limit. This can be a potential life saver if you use payroll. You’d hate for your test company to eat up what’s left of your available active employee licenses.