"END-check" isn't as "easy" as that because Arduinos (at least the UNO) only allows for 2 digits after the comma to be transfered over the serial. And adding a char to the end wasn't as easy as I thought (....try to convert a float with no special length to a char array/String... frustrating...). So I gave up on the "End-check".
None of this is true. When the argument is a float, you can supply a second argument defining how many characters after the decimal point.
You could use println() instead of print(), to automatically append a carriage return and line feed - they are good end of packet markers.
You could use dtostrf() to convert the float to a string, and then send the string, formatted however you like.
You could use more than one call to Serial.print() to send a start of packet marker, to send the float, and then to send an end of packet marker.
There are many ways to skin this cat.