The hallmark of any capable chef is that they can cook a steak to order. This can be tricky because the most desirable meat has a dark, crisp crust which doesn't give much indication if the inside is rare or well done. It's tempting to cut the steak in order to check the color of the inside and thus its internal temperature, but this strategy is actually unreliable. Luckily, there are ways to accurately assess the doneness of your beef without resorting to knives or even thermometers: using your finger.
As mentioned, it's not a good idea to cut into a steak to test for doneness, but not for the reason you probably think. Cutting the meat doesn't actually dry it out by releasing the internal juices. The cooking process itself (especially high-heat searing) reduces the water content of the steak, so there's not a lot of juice to run out. Apart from the fact that you're disfiguring somebody's dinner, cutting the meat will give a wrong impression of doneness because the juices that come out will make it look more rare than it actually is. Thankfully, there's a different and more reliable strategy that you can use that involves your finger.
If other options are unavailable, you've still got this well in hand - there's a decently dependable strategy available in your five fingers. First, press the top of the steak and see how much the meat yields under your finger. Open your other hand and relax your fingers. Different fingers on that hand touching the fleshy part of the thumb nearest the palm will convey different degrees of doneness. When pressed with the index finger of your other hand, the comparative softness mimics the resistance of a medium-rare steak.
Moreover, pressing with the middle finger represents medium, and medium-well is shown by using the ring finger. A medium-well steak should be firm with a little yield in the middle. When you cut into it for your first bite, the inside should have just a hint of pink. If this technique doesn't appeal to you, there's always the tried and true method of using a meat thermometer. However, using your fingers will save you time and money and still leave you with a perfectly medium well-done steak.
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