Learn Swedish: Ser du tecknet? - Do you see the sign?

The base word is ett tecken (a sign). When making an ett-word definite, you usually add -et. However, because tecken ends in an unstressed -en, Swedish drops that middle e to make it easier to say. So, tecken + et becomes tecknet.

How do I know from tecknet whether the base word is an en-word or an ett-word?

Look at the very end of the word! The definite suffix for ett-words is -et (or just -t when the stem drops a vowel, as it does here). Because tecknet ends with a -t rather than an -n, you can tell the base word is ett tecken.

Why does the sentence start with the verb ser? Where is the word for "do"?

Swedish doesn't use a helper verb like "do" to ask questions. To turn a statement like du ser (you see) into a yes/no question, you simply swap the subject and the verb. Putting the verb first makes it ser du (literally "see you").

Does tecknet refer to a physical street sign or shop sign?