Golden on the outside does not always mean safe on the inside. Chicken can brown before its middle is hot, and a burger can look done while the centre is still cool. Food poisoning bacteria are killed by heat, so what matters is how hot the thickest part gets, not how the surface looks.
A cooking thermometer with a thin probe is the most reliable check. Pick a food below, then slide to the reading on your thermometer. The tool tells you how far you are from the target. It follows two public sources, the UK Food Standards Agency and the US food safety agencies, and says where they differ.
What are you cooking?
Colour is a guide; a thermometer is the check. Heat the thickest part to the target, then rest it.
Bacteria such as salmonella and campylobacter are the usual cause of food poisoning from meat. Heat kills them, but only where the heat reaches. In a whole steak they live on the outside, so searing is enough. In mince, sausages and rolled joints they get mixed through, so the whole thing must get hot. Poultry and pork are treated the same way, cooked all the way through.
Time matters as well as temperature. The FSA lists several equal combinations: 70 °C for 2 minutes, 75 °C for 30 seconds, 65 °C for 10 minutes, or 60 °C for 45 minutes. That is why you may see slightly different numbers in different places. Aiming for 75 °C is a simple version that covers the common foods.
Push the probe into the thickest part and keep it away from bone, fat and the pan, which all read differently. Wait for the number to settle. In an air fryer or oven, check a second piece or a second spot, because the hot air reaches pieces unevenly. Wipe the probe clean between raw and cooked food.
Don't test food by touch, and don't rely on the timings on a packet alone, because ovens, fryers and thickness vary. Cooking more than needed also dries food out, so a thermometer helps you stop at the right moment, not just go longer.
If you want to try one, any budget works if it has a thin probe, a quick, clear digital readout, and can be cleaned properly. A reading within about 1 °C is good enough for home cooking. Rather than buying anything, you can also check against the cues above, but a thermometer takes the guesswork out.
This is general food safety, not medical advice. If you are pregnant, are older, have a weakened immune system, or are cooking for young children, the risks from undercooked food are higher for you, so cook meat all the way through and ask your doctor, midwife or a registered dietitian what is right for you. The same goes if you have a health condition or take medication. Anyone with symptoms of food poisoning that don't pass should contact a doctor.