All I have is a bag target (can't shoot broadheads into it). What do you guys suggest I use to shoot a few broadheads into to make sure their on target?

No you cant shoot b-heads into a bag target. Get a yellow jacket b-head target, they are awesome. I have been usingthem for2 years now and went through 2 of them but with b-heads you are going to tear up any target.

I buy the cheapo targets from Walmart. The 16"x16" ones that are 2" thick. Usually 2 come in a pack. They range between $15-20 and are good enough for me.

you CAN shoot broadheads into a bag target. It will be tough to get them out, but if your inserts are in solidly, and you have patience, the bag target will work fine if you have nothing else. I've done it.

A big pile of dirt

why cant you shoot broadheads into bag targets

I agree with some of the others. You can shoot broadheads into a bag target but you have to be patient in removing them. I wouldn't shoot a ton of them into it but a few, just to see how they are flying, won't hurt.

draw a target and put it on a hay bail

I agree with all the above posts. The yellowjacket, a bag a few times, or good ol' earth. If you can get in a tree stand and shoot at the ground, that works well also.