How would you know if you have a high enough sperm to have a baby?

Semen analysis. Will give you overall sperm count & % motile and % normal forms. Need > 39 million sperm per ejaculation, 40% should be motile & at least 4% normal forms. This refers to "natural insemination". You can still make a baby with very few sperm using invitro fertilization techniques. In fact even single sperm can be removed from a testis & "injected" into an ovum which can then be implanted into uterus.
Semen analysis. In today's age of advanced reproductive endocrinology, even men with almost no sperm can still conceive a baby through assisted reproduction. The first step towards determining a man's fertility is to see your doctor who can order a semen analysis and perform a physical exam and perhaps basic blood tests.
Fertility clinic. Drs klauber & kozakowski have good answers. I recommend getting semen analysis 'done right' at a reproductive endocrinology / infertility clinic rather than a hospital or general lab (labcorp, quest). Why? Using right microscopes, trained andrologists & quality control matter. Morphology (% of normal shaped sperm) has 2 standards, the 'strict' one where 15% is perfect and <4% is bad may be better.