I recently purchased a Ruger Ultralight and was looking at it today and think I've possibly discovered the beginnings of a hairline crack in the stock.
How should I determine if it is actually a crack vs a scratch?
In the picture it is on the left side of the end of the receiver running vertically.

Im not familiar with that particular gun, but couldnt you just remove the stock and look into the routed area to see if the crack continues beyond the surface?

I wonder if a drop or two of penetrating oil will reveal the crack... We do that at work on steel when inspecting for cracks, inclusions, etc...

Glue it if its a crack.

Having never tried my hand at stock repair how challenging is it for a repair like this?

If a goober like me can do it,read the destructions and cover lightly.

On vintage military bolt action rifles, we relieve the stock area around the receiver tang of wood to prevent cracks like that. Not sure you could do that in this case. A lot of guys use super glue (cyanoacrylate) to repair splits as it flows deeply into splits/cracks. I've never personally tried it. If that is a crack, it likely needs to be isolated and repaired due to its location. Just my 2 cents...

Have you done any stock repairs before?

Duct tape.

No no, JB Weld to fill the crack with Duct tape on top and a couple self taping screws to hold the duct tape down.

The best way to permanently fix it is to take two 2" wood screws and screw them in towards each other. Then take some two part apoxy and liberally coat the entire area.

Then Wrap in duct tape.

Take it apart and relieve the tang area on the inside so the action isn't acting line a wedge to split it. Don't get crazy and make a gap that will show from the outside,just make sure the action isn't what is splitting it. Put some accraglass, not the jel, in 2 jars like baby food jars and heat it in some boiling water until it is really thin. Use the heated epoxy to fill the crack. If the crack goes all the way through so you can get at it from the inside you can take a razor blade and cut the inside a bit wider to hold more epoxy.

Don't put any oil on it at all! In fact clean the area several times with acetone from the inside before you glue it.

Pour the epoxy on the crack from the inside but you will have to tape the outside and build a dam on the inside to keep the epoxy where you want it. The tape over the crack on the outside keeps any that gets through the crack from running onto the stock. The little dam inside keeps it on the crack and off everything else. When you get done clean up the inside with a dremmel.

When you are mixing the epoxy get both parts hot before mixing together. It will start to set up fairly fast after you mix them together.

RH beat me to it...It looks like a crack to me...He also gave you some dang good instructions for a fix though. I'm gonna hafta remember that fix too!

Bedding the front recoil lug will help keep it from splitting anymore too. The action is moving rearward and bedding the lug will stop that.

The tang should never touch that part of the stock. IMO the in letting is wrong allowing the tang to put pressure on the wood. I would remove the stock and sand a little to ensure it doesn't touch anymore and it should fix the issue.

RH Clark gave good advice on this. I have some epoxy repaired stocks. That's a great way to go, and a solid fix. I've repaired a number of military stocks, but nothing fancy like a hunting rifle stock.

You just have to plan your attack and work fast. Relieve that wood around the tang first. I'd do the lower part where it is cracked and up past that action screw if it were my rifle. When I do mine, I use a dremel with a fine sanding cylinder. A dowel with some fine sandpaper wrapped around it would be another good way to do it. It is not hard to overdo it with a dremel.

RH beat me to it...It looks like a crack to me...He also gave you some dang good instructions for a fix though. I'm gonna hafta remember that fix too! grin
Yea, like you need any help.Thanks though,

Bedding the lug is a great suggestion too. Might as well do it since you will have epoxy left over. Just use plenty of release agent on the metal and don't use so much epoxy that it overuns and creates a mechanical bind-lock elsewhere.

I might fix the crack first though and then get everything cleaned up before doing the lug. You wouldn't want tape or anything holding you off when you bed the lug.

Make necessary repairs.. Then replace with a synthetic stock

dont brownells have some videos about that?