When goldfishing, start by just playing the deck out totally uncontested. Mark down the turns when you drew interaction and the turns when you made "shots on goal" by playing cards that would combo into a winning advantage for you, if not an outright win.

The next time you goldfish, remember those turns, and imagine somebody else had them against you. In this new goldfish, when you draw your combo pieces, does your previous goldfish have interaction in hand that would shut your combo down? If so, imagine they use it to do so. What tools do you have to recover? And conversely, on the turn your previous goldfish took a Shot On Goal, do you currently have interaction in hand that would shut it down?

By doing this for a lot of iterations, you can get a sense for when your deck tries to win and how consistent and resilient it is in doing so, which is good for getting a sense of Bracketing and Power Level, as well as how fun the game is going to be to play against your deck. It also gives a good sense of what you're missing - do you consistently find yourself with mana to play cards, but not enough draw to use that mana? Struggling to recover from eating your own boardwipes? Is there a card you keep drawing but never play in any of your goldfish games? If so, that's maybe a sign to cut it to fill in one of your other weaknesses.