Can I use my Interrail pass on all Finnish trains?
It's worth checking in advance which trains have a surcharge or require reservations. There are certainly high-speed and night trains in Finland.
Most high-speed and night trains require a reservation at an additional cost.
Source: http://www.interrail.eu/interrail-passes/one-country-pass/finland
How can transit officials see where I've used my ticket?
There's a written record: on days you wish to use the pass, you make a written note on it; only then does it become a valid substitute for a normal ticket.
Fill in your personal details and ticket details before you make your first train journey. For every train, bus or ferry you travel on, you must fill in the journey details before boarding. Make sure to fill in your journey details before boarding the train! Read the full conditions of use of your Interrail pass.
Source: http://www.interrail.eu/passes/how-use-your-interrail-pass