The most popular way into the table search would usually be a screen field... Simply ask the end-user to point out which field is required on the screen, and take it from there: press F1 for help and get technical details via the "Tools" button.
You must have noticed that screen fields don't always lead to their physical tables directly. But there is a relation ! The developer at SAP does generally the same as you when creating an application.
He/she would like to put a field from a table on the screen, without having to work with the table-header information in ABAP memory. So another field or usually (DDIC) structure field is used. Whenever the SAP developer creates this structure, to data elements of the original table are likely to be referred to. And since this original table is the one you are looking for, try to find out what data element is connected to the screen field, and ask the system for a where-used list. This list can become pretty big, but one thing you can be sure of: your physical (transparent) table is there too.
If you would like to check whether a field value is valid, check tables are what you are looking for. From transaction SE10 one of the main tabs is about these tables. Alternatively Domain values can be the driving force behind checking values.