You may not be able to tell subtle differences in the high from harvest timing yet. The only way to know the difference is through experience, and that is nearly impossible to get if you are buying the buds.
Nearing harvest is the perfect point to start gaining that knowledge-- harvest in stages and decide which you prefer. The proper way to do it is double-blind:
Harvest a similar amount of bud every 5 or 7 days. Let it dry and cure the same way until they are all similar in feel. This would take two months or more if you do it properly.
Have someone else that doesn't know the significance take the samples and label them (A, B, C, D, E) in random order. Whenever you want to smoke, have THEM give you a sample without telling you which one they gave.
Smoke it, note the effects. You might not notice the difference until after most of the high has passed-- do you feel like you need a nap or do you simply feel fine and sober?
Once you have noted the effects, give your response back to the tester so they can file it under the proper label (A, B, C, etc). You should never know which one you were smoking.
Once the results are in, match up the original harvest dates with the anonymous letters and analyze your test. This also works well for seedling sisters to find the best mother among them.
If you don't have the patience (or tester) to do the proper method, you can still harvest in stages and decide which you prefer.