See Blues basics for a lesson/tutorial about this test.
Set a metronome or a very simple 4/4 drum loop to a nice slow tempo like 80 BPM.
Play 4/4 bars of quarter notes, but only play a chord on beat 1 of each measure.
Set a kitchen timer to 20 minutes.
Now play through several cycles of a full 12 bar blues in A with a quick change in bar two:
A7 | D7 | A7 | %
D7 | % | A7 | %
E7 | D7 | A7 | E7
Record yourself
Try to play as musically as possible, but keep it simple. As demonstrated above try not to do much more than perhaps hammering onto the M3. Only play the chord on the first beat of each measure. It’s okay to play additional beats (or even slide from a fret above or below in the last measure) but try hard to keep it as simple as possible.
You can repeat the exercise above three times in a row without making any mistakes or losing your place.
Listen to the recording (listening at 2X or 3X speed can be revealing). Does it sound musical? What suggestions can you make to yourself?