Blood tests and staying safe

Before you start taking hormones, you should get a pretty complete baseline set of blood tests:
 * Comprehensive Metabolic Panel (CMP) measuring liver function (so you can rule out liver damage caused by HRT.)
 * Testosterone (Free)
 * Testosterone (Total)
 * Sex-Hormone Binding Globulin (SHBG).
 * DHT
 * Estradiol
 * FSH (indicates activity of the HPG axis)
 * LH (indicates activity of the HPG axis, along with FSH.)
 * IGF-1
 * Complete Blood Count (CBC) measuring blood and immune system health (rules out illness causing chart weirdness or autoimmune reactions.)

Once you have this baseline, you know what's "normal" for your body. Don't assume you're starting with "normal" ranges; trans people are more likely to be intersex than cis people and many don't know it.

Every month (if you can afford it) or as soon as you've been at a normalish range for a couple weeks, get the following blood tests done:
 * Estradiol
 * Testosterone (Free)
 * Testosterone (Total)
 * DHT
 * FSH
 * LH
 * IGF-1
 * Prolactin (if transfeminine) (guards against prolactinoma, an occasional side effect of HRT that may cause brain damage.)

If your goal is binary transition, success looks like keeping these numbers in normal ranges for the sex you're transitioning to. If you're taking sex hormone blockers alone (at any age, even past puberty), you'll want to keep estradiol, DHT and T down to pre-pubertal levels. If you're not using bioidentical estradiol or testosterone directly, but some synthetic sex hormone (e.g. Tamoxifen, Trenbolone, be aware that the levels of estradiol and testosterone and DHT you get from these tests will mean little to nothing.

Running your own blood tests
Blood-testing websites

Here's a list of sites:
 * http://www.privatemdlabs.com/

Life-Threateningly Important Figures
You should check these levels regularly. Bad readings could indicate serious injury or death. Some of these figures (N) should be checked every time, others (2N) should be checked every other time etc. Defer to your doctor always!