I love Indian food and I cook and eat it often. I also have an Indian son-in-law.
Usually Indian places aren't difficult to eat low carb as they don't traditionally put either flour or sugar in sauces (if they use a thickener, it's usually a small amount of nuts) - this is more true of places that cater to Indians and have mostly Indian customers rather than more westernized Indian food. Most of their menus will describe the item and the ingredients that are in it.
Generally north Indian is easier than south Indian. South Indians favour a lot of dishes with rice in them (especially vegetarian places who rely heavily on rice, lentils and other grains. Hyderabadi places are easiest of all south Indian as they usually have kabobs and meat curries. Chettinad places also have lots of meat dishes).
Dishes I'd recommend:
- tandoori chicken (it should never have any breading) or other Tandoori dishes
- butter chicken*
- chicken tikka masala (wet or dry)* (neither of these dishes traditionally have either sugar or flour in them but it's safest to ask in case someone decided to get creative in the kitchen).
- egg curries
- fish curries
- meat curries ie. rogan josh (lamb), most meat will generally be lamb
- kabobs (if they are ground meat ask if they put flour or besan- chickpea flour in it, if they do avoid them unless you know you can tolerate besan/chickpea flour).
- vegetable curries without potatoes (aloo), ie. aubergine (baigan bharta), okra (bindi), lots of others
-paneer (a soft cheese) dishes like palak paneer (spinach paneer), saag paneer (mustard greens paneer)
- raita or curd (yogurt, helpful if you get a dish you find too spicy)
- chutneys and pickles are usually fine
- pakoras and bajjis can be iffy (some people react to the besan/chickpea flour breading, others don't), if you do have them, avoid the tamarind dipping sauce that sometimes comes with these, it has palm sugar (jaggery) in it to counteract the sourness of tamarind
Avoid:
- lassis (milk based drinks), mango or pineapple juice
- any bread (naan, chapatis, paranthas)
- samosas (wheat pastry & potatoes/peas), some versions have meat in them)
- dosa (especially masala dosa filled with potatoes), idlies, vadas, upma, uthapam, sambar, (these are south Indian). Stay away from semolina, millet, rice flour.sorghum.
- any dals (dried beans) & chana masala (garbanzo beans)
- rice and biryani
- kormas unless you know they aren't sweetened (traditionally they aren't supposed to be)
- vindaloo (not it's not just any hot curry. Traditionally it is made with vinegar and does contain a bit of sugar to counteract the vinegar)
- Watch the for rice flour used as a breading in deep fried snacks/appetizers. The following south Indian dishes often use rice flour as a breading which spiked me once, ask first : lollipop chicken , chili chicken and chicken 65.
- any desserts
- any sweets
-skip the multicoloured fennel in the little bowls for a post meal digestive some places have at the exit (it's coated with sugar), regular fennel is fine.
Ask for your masala chai (spiced tea) to not be sweetened.
BTW the word "masala" just means "spiced" in Hindi.