Thankfully, there are many vegetarian options available in most restaurants and fast food outlets these days. Usually they are on the main menu and some places use a symbol to make them easily recognised.
You do have to be careful, because even these days some folks don’t understand what “vegetarian” means. Fish is often considered vegetarian, oyster and fish sauce can be in curries, cheese often contains calf rennet, wine can contain isinglass (from fish swim bladders), etc. Make sure you check with the waiter if you’re unsure.
See Hidden Ingredients.
It can be a good idea to check the menu online, or phone ahead of time, especially if you are looking for a vegan meal. While vegetarian meals are commonly on menus, vegan meals often are not. Good chefs will be able to whip something yummy up, but generally appreciate knowing in advance you’re coming. Be sure not to be too difficult to please and thank them profusely afterward. This helps to encourage a favourable opinion of vegetarians and ensures they’re inspired to try again.