50+ parents want to get covid vaccine but do not go to doctors regularly so there is no way to know how they might be affected. any advice? they wish to get a vaccine tomorrow, even though i advised going to get a checkup first?

It's great your parents have access to the vaccine! They should do it, with no need to see a doctor first. There are almost no medical reasons not be vaccinated -- no matter what health problems they have, they should be vaccinated. The only minor precaution is that if they have had any other vaccine recently, they should wait 2 weeks before the COVID vaccine. Good luck to them!

Don't know what state your parents live in. The rules for vaccination may vary by state. There are very few contraindications to getting any of 3 vaccines available in the US. There are also very few times that a person would need to be medically evaluated before COVID 19 vaccine. I wish them luck in getting vaccinated.