Hi
I find even 0.25 makes a noticeable difference for me. To find out of
there is astigmatism:

1. Hold your glasses out in front of you and look through one lens
with one eye.
2. Rotate them so that the lens turns through 90 degrees clockwise
then anticlockwise still looking through the lens. The frames will be
vertical (like this : ) instead of horizontal ( . . )
3. If there is astigmatism you will see the image getting narrower or
fatter as you turn.

If you have the prescription then the astigmatism will be a second
power figure followed by x A
where A is the axis between 1 and 180 degrees. So if the prescription
for one eye is -2.50 -0.50 x 180 you have 0.5 dioptres of astigmatism
in that eye.

There is another format for writing the prescription like this:
-3.00 +0.50 x 90 which is exactly the same prescription just a
different way of writing it (seems to be less common though these
days, at least in the UK).

Richard