Where do I check the coolant at on my 1990 yamaha FZR 1000

There should be a coolant overflow bottle behind the side cover on the left side of the bike, near the seat. This needs to be filled to between the low and high marks. You also need to ensure that the radiator itself is full. The cap is on the top of the radiator on the right hand side of the bike, underneath a plastic cover from memory. Fill this right to the top.

All this needs to be done when the bike is cold to get the correct settings.

Nice bike by the way, there aren't many other bikes that perform so well regardless of what kind of riding you do on it.