Difficult to compare a trainer to even the same trainer used by another because of variations in set up. Different bikes, tires, tires sizes, tire pressure etc. effect the real and perceived resistance. Even more variables are introduced when the bike is taken outside, ...the terrain, wind, weather, lights, riding position.,.

A better way to compare across platforms is to measure how hard you are working at a given level of pedal revolutions. The cheapest way to get some basic feedback would be with a bike computer that measures pedal revolutions per minute combined with a basic heart rate monitor. If you are spinning the pedals at 90 revolutions per minute and your heart is beating at 140 beats per minute, you are doing roughly equivalent amounts of work, whether indoor or outside.

Choose a setting on your trainer and note your heart rate at a given pedal per minute revolution. Then note (the terrain, the gear, the conditions, the position on the bike) what it takes to recreate this heart rate at similar pedals per minute while on the road.

Your heart rate gives a somewhat clumsy measure of how hard you're working,...but its a lagging indicator (has to catch up to what you were actually doing some moments before) and also unfortunately varies with your physical condition - its still useful though. Most would say that measuring the watts generated is a better tool, but such facility is more expensive (a lot more).