Here is a list of essential equipment if you are serious about weld quality:
I am not affiliated in any way with these companies, but I do buy from them and they are all good people.

Tip force gauge - Don't be fooled by a slow gun, or the high "impact" of a fast gun, both give bad readings. Sensor Developments (goto "press releases" then "Archived" then select "Jun-01-00"),
or Tipaloy Scroll down near the bottom of the page for the force gauge

Current meter - Even if your controller displays secondary amps, it can't tell where that current went. Miyachi, Hover over "Products" and from the pop up box, hover over "weld checkers", then select the one you want, my favorite is the MM315A. or Welding Technology Corporation: Medar-Technitron-Robotron-Weltronic

Combination current and force = Squeeze analyzer, so you can see if the gun is fast enough. Miyachi, Hover over "Products" and from the pop up box, hover over "weld checkers", then select MM380A. or Welding Technology Corporation: Ask them for the Medar "Diverse" Squeeze Analyzer

Micro-Ohmmeter - Finds bad secondary cables, and shorts.
Roman(Listed as "Impedance Meter"), Biddle, Omega, (seems to have discontinued theirs), Simpson
PLEASE, if you find one that is less expensive, email me and I will add it...

Temperature sensor - Measure the tips 5 seconds after a weld, should be cool. Raytek. (get a 30:1 Spot ratio) They do have an 8:1 Spot ratio unit for $79, but you must be within a few feet to get an accurate reading.
Harbor Freight has one I use search for 96451-1VGA on this page.

Micrometer -You must know WHAT you are welding to apply the right settings. You can use a really cheap one, but most welding charts are in metric now, so try to find metric. A 1 inch outside micrometer is ideal, and if you can find one with pointed tips, called a point micrometer, it is even better. Remember the metal thickness often varies by 10% so your accuracy is not too important. Typical micrometer accuracy for a 0-25mm unit is .004mm, you will be reading .8 mm to 4mm and require only accuracy to .05mm !

Weld Checker Tool - This is WAY better than beating on a screwdriver handle, less work, safer, and you look more professional.
Cellar Services Weld Quality Tool These are often available in your plant's "General Stores".