Nobody wants to be the one who asks whether Dad should still be driving. The good news is that you do not have to make that call tonight. You can start by noticing the small things that tend to come before a bigger problem, write them down, and bring them to the people who can help you think it through, including Dad himself.
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The small signs first
Look at the car, not just the driver. Are there new scrapes on the bumper or the mirror, or a dent in the same place as last month? Is the car parked oddly, too far from the curb or across a line? Has he been honked at more often, or has he mentioned a near miss and laughed it off? Look at his route, too. Does he take a longer way somewhere he has driven for years, or get lost on a road he knows well? Write each sign down with the date you noticed it, so you can see a pattern.
Ride along, do not interrogate
Offer to come along on a regular errand, such as the pharmacy or the hardware store. Pay attention to lane position, how he handles turns and merges, whether he reads signs in time, and how he reacts to other drivers. Keep the conversation light. Mention what you see as a fact, such as the car drifted near the line, rather than a verdict about him.
Ask the doctor the right questions
Ask his doctor whether any of his health conditions, his vision, or any of his medicines should change how he drives. Bring your list of signs. Ask what the doctor would want to know or see before advising him on driving. The doctor's view is one important piece, and it is not the same as a decision made for him.
What to do tonight
Write down the three most recent driving signs you have noticed, with rough dates. Then check the car for new damage and note where it is. Do not take the keys away or make a plan at night. Plan one daytime ride along instead, and pick a short route he knows well.
Keep the talk about the car and the route
Start with the car, the route, or the errand, not his skill. Ask what he would do if he could not drive for a while, and who he would call. Check with your state licensing office what, if anything, it asks of a family in your situation, and ask a qualified professional if you need a formal answer. Then plan the next step with him, not around him.