True. Hence my caveat of “most cards”. If it’s got LEDs on the port, it’s quite likely to signal which speed it is at with those LEDs.

I haven’t yet come across a gigabit card that won’t do 10Mbit but sometimes I’ve come across cards that fail to negotiate speeds correctly, eg trying for gigabit when they only actually have a 4 wire connection that can support 100Mbit. Forcing the card to the “correct” speed makes them work.