The best way to test the salesforce data entry is to have a custom field created in salesforce - e.g. MarketingTest - and then create a journey where the entry criteria is based upon all your criteria + the MarketingTest = True. This will allow you to test the configuration, and then at go-live remove the marketingtest field.
In order to trouble shoot why it doesn't enter - there are many places to start. First of all, your Marketing Cloud Connect integration user - does it have the system administrator profile inside Salesforce Core? If not, it might be missing some permissions for the data.
Assuming that the integration user has all the necessary actions, you should be able to modify a record inside core salesforce, to meet the criteria, press save and then within 5-10 seconds, you should see that record in the data extension. If that's not the case, it means that the entry event might be wrongly configured. This is highly sensitive to having the right lookup fields populated, and ensuring you select the right entity (lead, contact or person account) to enter the journey.
Finally, if the data flows into the data extension, but not into the journey, you should be able to see why it didn't enter based on the entry event history (or journey history tab). It should say that a contact was evaluated and didn't enter. A typical use case here could be error to bind email, which has to do with the data you forward and your population configuration.
If neither of the above scenarios solve your issue, could you maybe share more insights about your scenario, what you are doing and where you are experiencing the errors? :)
