Can a midwife tell how far along the pregnancy is by palpating?

You can get an idea, but not an exact date that early in pregnancy. The uterus is rather small at that time and there are other factors to consider. First pregnancies may grow slightly smaller or "right on" for dates, whereas 2nd, 3rd, 4th, etc. time moms may have much a much larger uterus up until her third trimester. Also, a woman can have twins and make the uterus larger, some women have retroverted uterus'. A woman with a full bladder can cause the uterus to rise up higher in her pelvis, making the midwife think she is further along, etc. So, it's not really a cut and dry answer to your question.

We can get an idea generally by where you are measuring, but measurements do not start to become accurate until the middle of the second trimester. An ultrasound is going to give a more accurate dating than simply palpation at that juncture. Another option to consider though, is getting blodwork drawn and getting a quantitative HCG to measure hormone levels. That, combined with palpation, may give a better picture of where a woman is in her pregnancy. Again though, an u/s is going to be more accurate in the first trimester.

Not a midwife, learning. But this is something that interests me. There are signs at certain points in pregnancy that are pretty tell tale, but not so much in the first trimester. Palpation around the suspected time of 20 weeks is supposed to be pretty good, the uterus at that stage changes shape, at 20 week it should be at the belly button and round, it will no longer be taller than it is wide as it is earlier and will not yet be wider than it is tall as it is later.
I feel that if you are unsure of dates, it isnt critical to knwo so early and you can watch for all the signs and then put them together as time goes, such as first fetal movement, fundal measurement, palpation,also change in color of the vagina and cervix usually happens right as a women is entering the beging of the 2nd trimester.

As I said, no hands on experience at this point, just things I have learned and esperienced myself.

yes I could tell with an internal exam with one hand on the top so you are feeling the uterus between your hands- but that is with practice - I think that for women unsure of dates- like Apricot says those early ultrasounds are very accurate and something that is acessable to most providers- not everyone does or wants an early internal exam and I have to say that because so many women will have early losses that not doing an internal exam helps to distance a provider from suspicion of having "done" something that could cause the pregnancy to end --
there are later signs that can help to date a pregnancy- like when do you first hear a heart rate with a doppler and even later with a fetal scope- then later does that match up with fundal height at 20 weeks -- not having accuracy in dates can be problematic - if you guess wrong then what may seem like labor is too early is actually closer to term and ok or concerns about post-term when you aren't really there yet or issues with growth...

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I don't require my clients to have an ultrasound for early dating, but if they are not sure of their dates then an ultrasound is the only way to get an accurate date. It's their choice, but not having one is choosing to risk unnecessarily getting transferred out for preterm or post-dates. When you're working with a strict 37-42 week protocol, even one day can mean the difference between home birth or last-minute transfer of care. If you wait until the second trimester you may be close, but not as close as early ultrasound. Due dates cannot legitimately be changed in the third trimester to suit your desire to avoid intervention.

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I wanted to stress that this is an internal exam, not just abdominal palpation.