Special character in most font families. There's lots of way to find it but one quick way is to just hold down "alt" and start clicking through all of the keys until you find the check mark. Delete the rest of the special characters you just typed and copy & paste the check mark where you want it.
You can also find check marks in special fonts often suffixed "ding". For instance, Wingdings 2 has both a nice check mark and a check mark in a box and Zaph Dingbats has a couple of nice check marks too. Use "Font Book" in applications to quickly see the fonts you have that are symbol oriented. If you see a check mark you like, copy & paste it into Pages.
A third way is to do an image search for a check mark. Find one you like- ideally very large in size- and download it. Then drop it on top of a Pages document and size it down to the size of the desired checkmark. Drag it to the spot you want it. If it's background is not transparent, use "instant alpha" to delete the background color.
If you know how to locate a check mark but the question is about putting it in a box, use a text box in Pages so that you can have text on top of text (or anything else). Put the check mark in the text box (think of this as a layer) and drag the text box (layer) to the spot so the check mark is over the check box (as you want it).
I often get PDF forms that need to be filled out. When I want them filled out with type instead of by hand, I'll drag the PDF onto a Pages page, size it up to the right size, lock it and then use text boxes to align text to fill out the form within the spaces in the form. For check boxes, I'll either use the tip(s) offered above or just use a capital X.
You can place special characters anywhere on a document by using Edit > Special Characters, or press Control-Command-Space. This brings up the Special Characters window... click the >> slider and select the snow flake icon (Bullets/Stars), and then choose the check box you like. There are several available under the other categories too. ☑︎ ✓ ☑️