After 250 rolls, 1 popped up 27 times. Or roughly 11% of the time.
I did this same test with all 17 D20's I have and 2 D20's from people in my DnD group.
This one was far and away the worst though I had more than I'd like with averages under 10.
Edit:
Here's the stats for the ones I tested
This excludes the data for the 2 D20's from the people in my DnD group as they didn't want that data shared outside of the group chat.
A set of 250 throws with average of 9.56 in a fair d20 has a probability of about 0.5% to occur, so in 17 sets there is about a 8% chance to happen if you dice were fair according to binomial distribution. This means there is a 92% chance that your die is unfair. usually a confidence of >95% is needed for science but for home i think is enough to declare a dice unfair.
This calculation only says if the die is unfair, not how much unfair.