How do you gauge the safety of Indian street food?

Any new country will present your body with new and exciting microorganisms it needs to get used to. My first week in the US was miserable because I didn't expect it, and my first week in India was manageable, because I knew what was coming. The only country I didn't have any issues in was Japan.

The key is to gradually acclimatize yourself and manage exposure. For me, that meant bottled water during the first train ride, and only switching to tap water once I was checked into a hotel room with ready access to a toilet that no one would object to if I stayed there for a while.

The same for street food: venturing out of a hotel room to try things in the vicinity works well, trying stuff in transit between cities, not so much.

After a week or so, you should be used to it, but you might need to repeat this if you travel longer distances. During my second trip, I had no issues refilling my water bottle from the tap at train stations or buying food from mobile vendors there.

You should always have a water bottle with you, and it is a good idea to carry a small bottle of liquid soap -- public toilets generally don't have any.

Other than that, bring a few packs of electrolytes, because that is what you are going to be missing if you get diarrhea for any reason, and go with paracetamol instead of aspirin for headaches if you're in the malaria risk latitudes (Delhi is pretty much in the center of that band, and that is likely your port of entry).

I have been to India twice in the past and both times kept eating street food with no issues. While food in hotel restaurants was good, it was not even close compared to street food I would buy from a cart across the road from the hotel.

Note that I am used to Indian food and very spicy foods in general. I can eat raw green chilis without much issue (which I did to the amazement of locals in Hassan in Karnathaka where I attended a local wedding).

While India is huge and different areas have different traditions and different foods, I had a simple rule when considering street food: only buy from places/carts where lots of locals buy. For example, with two carts on a street, if many locals are buying from one and very few from the other, I would buy from the one with lots of customers. The other one may be even better, but I certainly didn't want to risk it.

Over two trips, I spent a total of about a month in India (Andra Pradesh and Karnathaka) - and never had any problems.

There is one easy way how to tell what to eat and what to avoid - see what locals do. If you see street food with lots of locals around, it's probably fine. If there are none, they probably have a bad experience with the place, so you should stay away as well.

You should also have proper vaccination when travelling - standard vaccination that is compulsory / recommended in your home country, as well as some "travel" vaccination (like typhoid fever, hepatitis A, rabies or so). Check with your travel clinic for details, as it depends on the destination.

Carry some hand disinfectant and have travel first aid kit on the hostel with activated carbon (this is not really a precaution, but it can help with consequences of bad food).

As for the tap water - don't bother trying to "acclimatize" unless you are going to stay in the country long-term. Instead drink bottled water or carry a water filter (I prefer Sawyer) and filter the tap water before drinking it. And avoid ice in your drinks.

One thing to remark is that people respond different ways to new organisms, so that there's a good chance that even if you follow the best practices described here, you'll become ill. It's really just the luck of the draw.

A travel doctor told me once that she prefers street food to a restaurant, since she can see the food being prepared and can get a sense for the food safety herself. That's reasonable advice, of course, but I would again emphasize the roll of chance.

The best thing is to maintain a good attitude and have a reasonable stock of medicines for traveller's diarrhea—which are probably best to buy in India, where they will be much cheaper.

Those qualifications aside, I really can't recommend the canteen at the New Delhi Zoo. ;-)

Food safety: Clean it, peel it or cook it.

Cleaning it is obviously not an option in this case. (And remember that tap water is itself contaminated and thus can't clean.)

Likewise, peeling is not an option.

Thus, cooking is the only way to ensure safety--only eat things which are freshly cooked and beware of possible contamination from utensils and dishes. Food safety in such places is difficult!