Changing transmission fluid-can a backyard mechanic do it?

I do a lot of work on my own cars. I started to change the transmission fluid in a recently purchased 1997 E320. I noticed two things: that there is no dip stick to indicate fluid level, and that
the fluid doesn’t all drain out. I took a guess at how much fluid I drained out, and added back in that amount. My questions to those who follow this forum: Is there:
A way to fully drain the transmission if working in the backyard?
A way to determine what the fluid level is in the transmission and compare that to what Mercedes recommends?
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