In principle I would always test twice or more on different days and in different circumstances.
Your example of tomatoes is probably not realistic, it would need a lot of tomatoes at one sitting to cause a spike. It would also be too tedious testing each food possibility in isolation. Better to find complete meals which do and don't work for you. Most people test immediately before a meal which might, for example, be their preferred 'go-to' breakfast and has everything that their breakfast would normally contain. So if breakfast is a cup of tea with milk and a biscuit, a boiled egg with one or 2 rounds of toast and butter and completed with a fruit yoghurt then test that complete meal at 1st mouthful and 2 hrs later to see the consequence.
I use that example because there are highish carb items that would probably cause a spike, then repeat without the biscuit or one piece of toast and retest, then drop the fruit yoghurt and retest - and so on. If that process always spikes despite checking permutations try a fairly different breakfast of omelette and meats, cheeses or mushrooms. Experiment to find what works for you and your circumstances until you find 'the' breakfast that can fit into your lifestyle. Then repeat that a couple of times to confirm your good result wasn't a flash in the pan.
This does all take time, but managing any diabetes is a marathon not a sprint. And once you are confident about 2 or 3 breakfast combinations you sort of bank those and stop testing those but start scrutinising lunches or dinners.
Welcome to the Forum, by the way. If you'd like to share a little more about your D such as how long you've been T2, what was the result of your most recent HBa1C and things you've tried that have not been successful members can tailor their response accordingly.
