When you google the correct amount of fork oil to add, it will probably come with a measurement, something like 191 mm or the like. That's the distance from the top of the fork to the oil, usually with the spring in and the tube fully extended. Go to your 1$ store, buy a spray bottle, remove the spray mechanism, cut the tube that extends down to the length listed. You should be able to insert the tube into the fork, and if you can spray any oil out, you've got too much in it, otherwise if there is no oil at all on the sprayer when you remove it you may not have enough. Despite the helpful crowd here, don't fill it to the top of your fork and replace the cap, unless you found the fork seal changing process enjoyable.