Are you concerned about whether a vial contains bad insulin? Insulin can have a diminished ability to decrease blood sugar levels compared to what was expected. There are lots of discussions on the internet about how to identify bad insulin, so I'll focus on what to do with the insulin and the vial when you suspect you have bad insulin.
I'd like to point out that there are two different approaches with bad insulin: first, contacting the manufacturer or your pharmacy about returning suspected bad insulin, and second, deciding to discard a vial of partially used insulin. However, before contacting the manufacturer or tossing the vial, it is important to talk to your health team to check if your blood sugar is out of control because other medical reasons. Bad insulin is not the only cause of high blood sugar.
Contacting the manufacturer or pharmacy
For many insulin products, the liquid that contains insulin should be clear and colorless. Therefore, if you open a box of this kind of insulin and find that the liquid in the vial is discolored, or that the liquid has particles floating in it, there's something wrong. In such a case, the insulin should not be used, and the manufacturer should be notified. They will want to take back the vial and test it, and they will arrange for you to get a replacement vial. For example, labels often state, "Only use insulin that is clear and colorless. If it is cloudy or slightly colored, return it to your pharmacy for a replacement."
If you are using a type of insulin that is naturally cloudy (such as NPH), you might find small white clumps that sank to the bottom of the bottle, or a white coating or "frosting" on the inside wall of the glass vial. Again, the insulin should not be used, and arrangements made to return the vial for replacement.
There's another circumstance about bad insulin that might occur and doesn't quite qualify as the manufacturer's problem: insulin shipping problems before the vial gets to your possession. For example, I was involved with an FDA-mandated recall for one insulin product a while back, where a national pharmacy chain was storing the insulin in non-refrigerated trailers in the heat of summer, resulting in lots of consumer complaints of bad insulin. Ever since that episode, I've been concerned about the possibility of bad insulin that had been shipped in the heat of summer -- or the cold of winter. And if you get your insulin from a mail-order pharmacy, it might seem more likely that the insulin could have been left outside in circumstances of extreme heat or cold on a shipping dock someplace or in a delivery truck. I realize this is a risk for insulin delivered to your local pharmacy, as well as for insulin delivered to your door, but it seems more likely the more steps there are in the delivery chain. If you open a new vial of insulin in mid-summer or mid-winter, and it just doesn't have the usual effect, maybe it was a shipping problem, rather than a manufacturing problem. Take it back to the pharmacy, or call the mail-order pharmacy. If they won't help, call the manufacturer - and mention the local weather.
Tossing the vial
There are numerous statements in the various insulin labels advising you to discard the vial "if". Let's break these warnings into several categories. Look for a section of the label titled "Storage" or "Storage and Handling" for details.
You'll see statements such as "Insulin should not be used after the printed expiration date." This advice is based on studies done by the manufacturer and shows that after a certain amount of time in standard conditions, that potency diminishes. If you are storing your insulin in the same conditions, you can expect your insulin to last that long, and probably longer, but the potency of the insulin will diminish with passing time. It's up to you to watch the effectiveness of your insulin, whether inside or outside of the manufacturer's time window, and if you choose to use insulin that's outdated, it may or may not go bad. This is true both for opened and unopened vials: the labels give advice for both situations. Use the vial with extreme caution if outdated.
Heat and cold:
Both heat and cold will damage insulin. Statements in the label such as "Do not freeze insulin and do not use if it has been frozen" and "Insulin exposed to temperatures higher than 98.6°F (37°C) should be discarded" should be heeded; I'd think the odds of bad insulin would be too high to run the risk. Toss the vial.
Some insulin products are not compatible with others. For instance, it's the official recommendation for some insulins that it "must NOT be diluted or mixed with any other insulin or solution." If you usually mix two insulins together before administering them, and have a one-time goof on what you did, don't give the shot. But the vials should still be okay to use unless you deliberately squirted one insulin type into the other vial; then toss that vial.
There are other situations that don't show up in the product labels that might encourage you to toss the vial.
The most common, I suspect, is the feeling that every time you start a fresh vial of insulin, that the stuff is more potent than the dregs from the previous vial. That might indicate that the previous vial (and every vial you use) is losing potency during the time you're using it, even if you are using optimal storage conditions. This is difficult to be sure of unless you keep meticulous records, but if you think the vials are losing potency rapidly, the safest thing to do is to discard the vial before it's down to the last drop. Exactly when to discard the vial is a cost-benefit decision: it's costly to toss the insulin, but the benefit of avoiding bad insulin may be worth it.
Bad insulin is a problem that will remain with us, and it's unlikely that anyone will ever be able to quantify how often it happens. But when it happens, it's not always necessary to toss the vial; it depends on the circumstances! Lastly, consider that your insulin might be still be good and consult your doctor to see if you might need to adjust your dose.