The Luftwaffe could be fairly certain that those hundreds of signatures coming from the direction of England at high altitude weren't Me109s though.
Current military radar can't even classify the type aircraft. There's still some mystery left, even with todays technology.
The Chain Home system used by the British relied heavily on visual spotters to confirm the position and type of aircraft in each mass return. Until the spotters V-ID'd the aircraft, only then did the RAF know if they were facing massed bombers, massed fighters, or a combination of both.
The Germans may have seen multiple blobs, but they couldn't tell you which blob was a group of fighters and which blob was a group of bombers from the radar return. They had to use other techniques to categorize what they thought the returns were. They studied the history of the return, and if it conformed to the profile that matched the bomber stream, they categorized it as a bomber stream, but even this was an imprecise means with which to identify returns.
Case in point: In Spring 1945, a few P-51 groups launched a massive formation of fighters that bunched up into a tight formation, and flew at typical bomber altitudes and speeds. The Germans launched a massive group of fighters to intercept what they thought were unescorted bombers. (I don't remember the specifics of the units, but IIRC, this involved Yeager's group so you could cross reference this with that, if you so desire).
As several have mentioned here, watching the clipboard map will give you an indication of the bombers approaching, if you look for the tell-tell signs, typical profiles, etc. Its not perfect, but it's a relative match for the radar capabilities of the opposing sides during WWII.
