What kind of fertility test do the dr use to determine why a person is having problems conceiving?

They will definitely check his sperm for quality and quantity.
They will check to see if you are ovulating. If they suspect you aren’t ovulating, they will run tests on your thyroid and pituitary etc.
Really not sure myself as I haven’t gone through the full round of testing. I was able to tell on my own by charting my BBT that I wasn’t ovulating and they ran the thyroid and pituitary panel which was clear so I’m taking my first round of Clomid right now.

They may do an ultrasound to check your ovaries for cysts and may want to see if your tubes are clear.
I’m not sure what they would test for differently in a 41 year old versus someone under 30.

There are a variety of tests – for your husband there would be a semen analysis. For you, there would most likely be a blood test around CD3 to check FSH (follicle stimulating hormone) which indicates your ovarian reserve / egg quality and quantity, there would be a blood test around CD21 to check Progesterone which indicates whether or not you ovulate, blood test to check other hormones like thyroid function, HSG which is where they inject dye and check your fallopian tubes for blockage and possibly an ultrasound to check your ovaries and uterus.

Good luck and baby dust!

The doctor will probably do a sperm analysis on your hubby. For you, he will draw bloodwork to see if your ovarian reserve is still good, are your hormone levels in check. They may do an ultrasound or HSG (hysterosalpingogram) to see if there are any blockages or abnormalities. Hopefully they can figure a plan for you to concieve from this information. Best of luck hun and baby dust to you!