Put an indicator on a holder, register the holder against the end of the v-block and set it up to indicate the V vertically. Clamp a second indicator mount to the side of the V block, and indicate the end of the holder. As you slide the first holder against the end of the block perpendicular to the V, the indicators will change by identical amounts if the block is exactly 45 degrees.
No idea how accurate or practical this might be.
Another thought is to scrape a triangular block with a 90 degree peak to match the V. The surface of the V facing up won't necessarily be level, but if you flip the test triangle around, the same spot on the triangle will only be at the same height if the V is at 45 degrees. More work, but probably a lot more accurate.