Ok,
Since I am coming from a programming background, I can definitely answer your question.
Technically speaking, *anything* is detectable given the correct amount of programming. If someone *really* really really really wanted to know if you were using a software tool to help you type in form details, then given enough $$$ and programming expertise, then yes, they can do it.
However, that is a lot of work, and most (educated guess) 99% don't do that, because it is not worth the time/effort when much 'simpler' ways are available to still making the sweepstakes fun for everyone. The "smart" ones just make sure people enter according to entry details (i.e., if it's once a day, enter once a day), and 'ignore' entries past that. (So you can't enter 10,000 times, when it is once per day, and programatically, that is relatively easy to do).
I've personally spoken with a number of different companies -- and there concern is not so much for automatically filling in a form, then clicking submit -- their main concern is:
(a) When someone tries to abuse a sweepstakes, i.e., enter 10,000+ times in a day when "regular" people can only enter once
(b) If the user entering NEVER hears/knows/or has access to the company (so the company does 'not' increase their exposure), then they are concerned about that as well.
Roboform has been around for 10+ years, and thousands of people have won prizes with that (just visit any
sweepstakes websites forum, read the old threads, and you'll see)... Sweepstakes Ninja, a new tool which makes it even easier, has been around and developed over several years as well. It has been designed with sweepstakers in mind, and also has many people reporting have won from using it. And it enters according to frequencies specified by the companies, as well as gives them the increased exposure. (In fact, I've personally received comments from people thanking me for letting them know about a company they would have never otherwise known about, that had some great products & services).
So bottom line:
(a) Ultimately it is your decision whether to use a tool or not. If you are concerned, then you should enter all sweepstakes by hand.
Most sweepstakers (according to forum stats) at the very *least* do use Roboform, which 'technically' could be considered a form filler, since it is.
(b) People have won using Roboform & Sweepstakes Ninja. Both let you 'see' what you are entering, while making the process more enjoyable. And companies like you being happy when you visit their sites. And companies like you telling other people about the wonderful prizes you've won from them, to increase their word of mouth advertising.
(c) And yes, if someone really really wants to see if you are using some kind of tool, there are a thousand different ways of doing that.
The ultimate thing to remember is, companies want to have sweepstakes, in order to increase their exposure, word of mouth advertising, (and ideally more sales, etc). If you are using a tool that helps them *acheive* that (i.e., you still see the website, know what you are entering, regardless of if it takes you 1 second or one minute to enter), then they are usually happy as well.
Johnathan