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I have a Strat that I left the strings off of for a long time and once I finally got it together I noticed a lot of buzzing. I'm assuming the neck is warped, but I'd like to make sure. But I've tried to look down the neck and stuff and I just can't tell if it's warped or not

A guitars neck is not supposed to be straight. It should have some relief in the middle. The best way and easiest way for a novice to examine the neck is to use the strings. Hold the 6 string down at the first fret and at the last fret. Check to see if you have any space between the bottom of the string and the top of the 9th fret. If you have no space the guitar will buzz. You should have a small amount of space. I use a piece of .10 gauge string and youshould be able to pass the string under the 6 string and over the 9th fret. If you don't not have enough space it will require a truss rod adjustment. Loosening the rod will increase the space between the fret and string.
If you have not done this befor it might be wize to take the guitar in to a service tech and have it set up. A guitars neck will move at different times of the year due to humidity or the lack of. A guitar should be set up a few times a year.

If the guitar is backbowed, the strings will usually hit at the first fret. If the guitar is perfectly straight, the strings will usually hit in the mid section (frets 5-10). If the neck is overly curved, usually it will just have overly high action and a very noticeable curve to the neck. Just my experience.

As IZ said, the back bow is the least desirable situation.

You can look down the fretboard, from either end, and see if the neck is twisted.

The relief is the other major issue. When checking the relief, check for high frets.

Don't discount the feel of the neck to you. If it doesn't feel right, don't buy it.

would there be there any other reason for the strings buzzing? it only happened once I put it back together after a few months of being unassembled

It is best to store necks with the strings pulling some relief into the neck.

I this case, you will likely need to tune the neck to pitch, adjust the trussrod, then tune again.

Because it hasn't had that stress from strings applied to it, it may need to sit a while with the stress applied.

....tune it and let it sit a few days.