I have some simple questions:

How do you know when a wheel bearing is approaching its service life?

How difficult are they to replace?

Are they costly?

The reason I ask is because I am starting hear a "whirring" noise from the front of my savage and I suspect it is a bearing.  The bike is stable during 50+mph but I have notice a bit wiggle on evasive manuevers.

Front brake pads make a scrape/whir noise... they are always in light contact with the rotor...  This is normal...

raise the front end and give it a spin. like Versy says...
... but you might want to pull the caliper first... (just two bolts)...
Then listen for noise...

Bearings are $5 or $10 bucks... pay more, and get Japanese or German ones, not from China, India, Malaysia, etc... they're worth the extra...

It's straightforward.  I have a bearing puller, which is a slide hammer with an expanding collet, but a lot of people just use a hammer and a long punch for removal.  For re-installation, use a socket of approximately the same size as the bearing and make sure to apply force only to the outside race of the bearings.

Hehehe...

back to the topic... without a bearing puller it can get complicated sometimes, depending on the wheel, and that spacer inside. Here's my trick:
Tack weld a nut or thick washer to the inner race, just to close the hole as much as you can. Turn the wheel, run a thick steel rod through the bearing on other side until it touches the welded nut. Hammer out. Turn wheel, aim other bearing inner race with steel rod, hammer out.
There's another effective way, no welding needed, but it involves some very controlled hammering (ninja-like control) with sharp chisel. I'm not going to even go there.

For install, freezing the bearing helps. You can use the old bearing to hammer new one in. Align race to race and slowly tap around. If it looks off or angled - STOP! Get it out gently and start again. (yeah, good luck with that). Once it's in about halfway, you can press it in to speed up.

You might lay hands on a replacement bearing & just feel of it, see how stiff it is to spin, you dont have to buy one, go to a bearing supply house & look at a few bearings, get familiar with how they feel when new,, just be nice,  maybe buy something,, I get Kroil & a few other shop supplies at the local bearing supply house,, they dont seem too irritated when I take up their time on little thiings that they cant make any $$ on.

side to side "play" in the wheel indicates a worn bearing