How do you judge when a fast solo is up to speed? Do you play along with the solo on the record? Use a metronome and see if you stay in beat? Find a the same part of the rhythm that the solo plays over and play over it? Record the rhythm at the right tempo and solo over it?
I usually just go by sense of rhythm. Try playing the song that you want to solo and playing with the speed of the drums. If you know the exact tempo of the song, you can practice with a metronome too.
I use a metronome. I also may go beyond the original speed, if I'm using the solo for practicing technique (which is the case more often than not).
(Slightly outdated) Electronic and classical compositions by m'self: Check 'em out
...just play with the solo...
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Work with metronome, play along with record, simple as that. If you can play along with the record with minimal mistakes, then you got it up to speed pretty good. Keep working on it until you can do it without thinking.

Remember, you may be able to play along with the record, but if you mess up every measure, it's not really up to speed.
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