Yamaha YZ125 1985 is the cylinder nikasil plated?

No. The first use of Nikasil by Yamaha on the YZ appears to have been on the 1989 YZ125.

One really easy way to tell is to use a magnet. Nikasil over an aluminum bore is non-ferrous where as the steel sleeve obviously is.

If the magnet sticks, it is a steel liner.