If you are going to do the nervous noob thing and worry that since you didn't see airlock activity or little fairies hovering around your fermenter, you don't have fermentation, then yes, if you really want to know what's happenning, then yeah take one around the 7th or 10th day.
If you are going to excercise some trust in the yeasties, and since you are already waiting 3-4 weeks before bottling, then you can do what I do, take a hydro reading on brew day to determine your OG, then again after 3 (since that's how long you said you were planning to leave it) weeks, if it's in the target range and you want to bottle it then, you have now figured out the FG of the beer, which you can use to determine the abv of the beer, plus attenuation.
If you realize what a hydrometer's purpose is, that's it's your diagnostic tool if you think something's wrong (which it rarely ever is) , that it is your fermentation gauge as opposed to your airlock- you use THAT before you cry "is my beer ruined" or "should I re-pitch"- Then use it on as
at needed basis.
I get down a lot of new brewers, because they think about reaching for a pack of yeast BEFORE they think or reaching for their hydrometer, when they start those kinds of threads. Actually using something other than their eyes or assumptions about airlocks to tell them what's going on.
By leaving your beer 3-4 weeks, besides all the reasons we have discussed on here ad nauseum, you are giving the beer long enough (usually) to finish fermenting, which a lot of first time brewers are too impatient to do.
You don't have to use a hydrometer at all to make beer (but that usually works out when you have a few batches under your belt, and enough experience to gauge what's happenning at any given time.
On the other side of the coin, you don't
need to use your hydrometer , every minute of the day either.
You just need to strike a balance, and know when you should use it.
