I would probably try something like the LT5504 (RF log power detector) if you need to detect a signal level from very weak to very strong. It covers the wifi range, and has a dynamic range of 70 or 80 dB.
The MAX2015 is similar.
You can use either (or similar parts) to drive an LED proportional to the RF level. You could also drive a small meter with the detector, and calibrate the whole shebang to show the real received power level.
Both cost around $10 US. May be too expensive and overkill if you just want an on (lots of RF) off (little RF) indicator.
The LTC5505 is an RF peak detector. You won't have the dynamic range, but that would be good for a simple on/off indicator of high/low RF level.
There are many similar parts from various manufacturers. I've just listed a few from my own research for an on-going hobby project.
You may also look into the circuitry used in detectors used in the little boxes they sell to help set up satellite dishes for TV reception. The circuit diagrams can be found on the internet in various places.
You might also consider just buying and using the cheapest of these gadgets - preferably one with a meter.
Satellite signals are around 10 to 12GHz, but are down converted to around 2GHz - right in the range for wifi.
Connect a 2GHz antenna to a sat detector and wave the antenna around - it should react quite well to wifi signals.
You'd probably want to modify them a bit if you plan on using them a lot. You'll want one where you can turn off or disconnect the 24Volts that normally power the LNB on the dish. With a regular antenna you run a risk of shorting the 24Volts, though you can get away with it if you are just checking to see if a sat detector is adequate to your needs.
Another thing is that they often use a version of the ancient 741 opamp to drive the meter - since the 741 output can't go all the way to ground, the meter never goes to zero. There are more modern opamps that are pin and voltage compatible that could be used to improve things considerably.
Finally, you will probably need an adapter connect the antenna to a sat detector. They have different connectors than the typical antennas for wifi. Adapters can be expensive.
Finally, to the antenna.
Yes, an antenna can be as simple as a piece of wire of the correct length. The trick is getting the correct length and having a good "ground plane" for it to work with.
There are many examples of wifi antennas on the internet. Some are good, some are bad. Take a look around, find one you think you could manage to build, then ask here (new question) if the antenna design looks good.
There are also antenna design programs that can help you calculate antenna part lengths for various frequencies. Try one for a simple dipole at 2GHz and 75Ohms impedance. The programs (and online sites that have such calculators) will generally also have at least a sketch showing how the antennas go together.
There is a lot to learn when you go to building a gadget to detect/measure RF.
You can use commercially available detectors (such as the parts I mentioned above) or build your own using discrete diodes like the satellite detectors do.