Tracks that beat Traktor's beat detection
Traktor's not always perfect in automatic beat detection. I was trying to set the correct beats per minute (bpm) value for a particular track and not having a lot of luck with the tap function that Traktor provides. The estimated bpm kept jumping around on each click.
If you're not familiar, a tap feature is common in music software for beat detection: tap a button along to the beat of the music, and after a few clicks, you should have a fairly good estimation of what the bpm is.
Another option you have is to manually adjust the beat grid to set the bpm; the beat grid is a series of line markers that you want to mark the track's beats. It acts as a visual guide for the user that shows whether the bpm is set correctly.
This is a chore as it requires fine adjustment. You'll end up clicking a lot to stretch or compress the grid and correct it. Also, if you're zoomed in to precisely align this, you can't see if everything's still OK further along the track without going back and forth, like Cameo.
Beatmatching to find the bpm
The last time I was contemplating doing this I realised that if you can beatmatch by ear, then to find an unknown track's bpm, you can simply beatmatch to a track with a known good bpm. It's more fun too as you're actually listening to the music and DJing, rather than click-click-clicking like you're gold farming in some video game.
Once you're happy that the tracks are beatmatched, if you have the current bpm for the known good track displayed — you can change what's displayed in each deck's heading in Preferences — then that should give you the correct bpm for the track with unknown bpm. You can then type this in where the bpm is displayed on the "Grid" panel for the deck playing the track.
All that's left to do is seek to the first downbeat and add that as a beat grid cue point (use the "Cue" panel for the deck) and the track should be ready for beatmatching. Traktor usually does a decent job on finding that downbeat though.
That would be all, except you might only have a correct bpm for the particular section you've been listening to: it's possible that the bpm fluctuates. If the beat detection fails, this is quite a common problem, particularly for older pieces of music which weren't created digitally and may drift slightly due to human timing. A fix for this is to just work with a loop in the track which does have a sufficiently steady bpm to mix. You can add loops in the "Cue" panel.
While I'm here, I should wistfully wish that Native Instruments would improve the beat matching aspect. Using multiple beat markers, Ableton's capable of adjusting — "warping" they call it — the track to fix it such that it has a consistent tempo, something that Traktor lacks. So, I'm still saving for Ableton.