Hi Tim,
I build several hotmail or gmail accounts which have "MKTGtest" as part of the email address (
MKTGtest1@gmail.com,
MKTGtest2@gmail.com, etc.). You can have other testers build their own test accounts, just make sure to all your testers that the email address names they define has the root "MKTGtest" in the email address name.
I run one of these leads through each of the creation modes (New Lead, import from list, Fill out Form) in order to see what the system (entire instance) does with a new lead. Once I have seen this activity (through view in Activity Log), I build filters in each of the affected trigger campaigns to ensure that any lead in which Email Address "NOT CONTAINS MKTGtest" is blocked from executing (eg., sync lead to SFDC, Change Data Value, etc.). This ensures the tester that the email campaign will behave as expected. Any abnormal point scoring or other situation can be dealt with at a later time.
After running your campaign to completion, and you are satisfied with the outcome, you can DELETE the lead from Marketo, so that the metrics of the leads which ran through your email campaign will not be skewed by all the test lead addresses. If you don't want to delete the test leads, just make sure that the query has the additional Email Address filter to "NOT CONTAIN MKTGtest".
Hope this helps.
Don
(Marketo Certified Expert)
(former Marketo Sr. Tech Support Engr)