Test dowels before glueup. Takes time, but well worth it. Depending on how and how long they've been stored (and manufacturing tolerance), they can absorb moisture from the air and swell slightly. Sort them into groups- Too Tight (for when you get a sloppy hole), Too Loose (for when you have alignment issues), and Just Right (hopefully most of them).

Hard to know what is Too Tight and Too Loose. The glue will provide some lubrication, so dry fit should be a bit tight. If you have to drive them in- too tight. If they wobble at all in the dry hole- too loose. Too-tight dowels can also 'telegraph' a slight bulge in the surface from the moisture in the glue swelling the wood dowels- especially if you apply too much glue and it has no place to go.

Also depends on the workpiece- a tight wood like Maple should give a good 'pop' when you pull them out dry, but plywood/MDF probably will not. Materials will absorb more/less glue, and that is a consideration.

Tomosakis included a link to the right type. I'd be a bit concerned about buying from a Big-Box store. No way to guess what condition they are in (they may be just fine). For that matter, hard to know from any source. Test, test, test. Don't try to redrill holes in your workpiece if you think you need bigger holes. That's what scrap is for.

Don't drive yourself nuts (or ruin a nice project) just because you were too cheap to throw out a batch of dowels that went bad in storage. Dowels can go bad, just like glue.