Refer to the frequency spectrum. If there is a space on your spectrum that is fairly empty, your track won't sound full. when using reference tracks as mentioned above, look at how that track looks on the frequency spectrum compared to yours. you can view the frequency spectrum with most EQs by enabling the analyzer. Metering plugins also usually have spectral analyzation modules.
So for example, say I'm lacking in the mid and highs by a few dB, what would you suggest to flesh them out? I guess it depends on the track and the sound you're going for, but any ideas?
It definitely depends on the sound you're going for. I would suggest just layering an instrument that occupies that space to your liking. To take the reference track idea further: you can use an EQ on a reference track to isolate those specific frequencies and analyze what type of sound occupies that frequency band. Use that as inspiration and ideas for your own work. It's okay to take notes from professional producers. They are professional for a reason.
Also explore the idea of using real instruments in your work. I know logic has synthetic instruments such as pianos, keyboards, guitars, and orchestral instrument patches built in. those instruments will sound good by default because they have always sounded good, and will also occupy a wide range of frequencies, giving you more to work with when it comes to EQ. and on top of that it will give your productions a bit of an organic sound which is always nice
Let me know if this helps