Lots of ways, and different ways for different font formats, for different programming languages/environments/font editors, and different levels of expertise.

One such way would be to open an associated .afm file with a plain text editor, find the string "Ascender", and divide the number right after it by 10 to get a value as a percentage. Will of course only work for Type 1 fonts, but on the plus side, it's so easy anyone can do it.

How about OTF and TTF?

Again, It Depends. I have no problem reading raw data straight out of the TTF/OTF hhea table, where you can find various measurements, but then again I know where to look for them.

What is your ultimate purpose? Your platform? Programming language or other way or means you intend to use?

Also: are we talking about the numeric value in the font file, or the actual height of the ascenders? Noting that (unless it's an RdR revival) you can't be totally sure of the latter.

hhp

If you have it opened in a font drawing program, you can just read the coordinates of the nodes. Otherwise, just blow it up on screen, and measure.

The only thing is that the typical slanted head-serifs can make it a bit fuzzy.

hhp

What I'm trying to do is align text to baseline grid in InDesign, but also have the ascenders touch the top of the text frame.

I think you can only do this reliably by going into the font. And if the ascenders have slanted head-serifs, you're going to have to decide where their visual top is.

hhp

inktrap: InDesign lets you choose the top offset for a text frame. See "First baseline offset options" on the online Help.

For this to work, you cannot use a document-wide baseline grid (unless you "manually" set its starting position). Since the position of your first line is going to depend on the ascenders in the first line of your current text frame, they may differ from frame to frame; and hence, the position of the baseline grid, which starts at the bottom of that same first line.

Store these settings in an Object style, so you can apply it to every frame where you need it.