To figure out what is currently in the system, scoop a little bit out of the master cylinder, and dump it into a clear jar with a quantity of DOT 3 in it. Then shake it up and let it sit until the next day. If your system has Dot 5, they will separate. If it has dot 3 or dot 4, they will stay blended.

I have been using Dot 5 in all of my collector cars and race cars for 40 years. I have put cars into storage for over 20 years, and pulled them out and they still had a perfectly working hydraulic brake system using Dot 5. I would never put anything else into my cars now.